The Sea Swimmer's Case for the Swim Brief
Best swimwear for cold water sea swimming: why the regulars at the Forty Foot all wear swim briefs, and how a two second change keeps you warm.
What to Wear Under a Wetsuit
A wetsuit works by sealing a thin layer of water against you. That single fact decides what should be underneath it, and it rules out most of what people wear.
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Every surf school on every coast has the same conversation in the car park, usually with a nervous new customer holding a damp 4/3 at arm's length. The instructor shrugs and gives a non-answer, the customer keeps on whatever they arrived in, and forty minutes later they are standing in the shallows discovering the specific misery of a cotton waistband soaked in cold sea water.
That misery is avoidable, and not as a matter of taste. It follows directly from how the garment works. Get the mechanism right and the answer falls out of it.
How a wetsuit actually works, which most people half know
Neoprene is a foam: polychloroprene blown with tiny gas bubbles, and those bubbles are the point. Gas is a poor conductor of heat, so a sheet of neoprene is very reluctant to let energy through it. That is the primary insulation and it does most of the work. Water is a far more efficient thief of heat than air, which is why you can stand about happily in a fifteen degree breeze and be in trouble in fifteen degree water.
The famous bit, the thin layer of water, is real but secondary and widely misunderstood. A well fitting suit lets a small amount of water in and then holds it there. Your body warms that film, and once warmed it becomes an additional insulating layer between your skin and the neoprene. It is why a wetsuit feels dramatically better after the first minute.
The critical word is holds. If the suit fits badly, or the seals at neck, wrists and ankles are loose, that warm film gets flushed out and replaced by cold sea water, over and over, all session. A flushing wetsuit is not far off wearing no wetsuit at all, which is the most useful thing to understand about the whole category. Modern suits add sealed seams and thermal linings, but all of it depends on the suit sealing against your body.
So the design brief for whatever goes underneath writes itself. Nothing that holds cold water, because anything absorbent creates a reservoir your body has to warm and which stays cold longer than the film would. Nothing bulky at neck, thigh or shoulder, because that breaks the seal and invites flushing. And nothing that adds a rubbing layer where the suit already rubs.
That is the whole theory. The rest is application.
Why a cotton brief is the worst possible choice
Hold a cotton brief against that design brief and it fails on every line.
Cotton absorbs many times its own weight in water and gives it up reluctantly. Put one under a wetsuit and you have introduced a sponge into a system whose entire operating principle is minimising the volume of water your body has to heat. The suit warms its thin film in a minute; the cotton stays cold for the whole session, against the parts of you least able to tolerate it.
It gets worse when you move. Saturated cotton sags and bunches, and a cotton waistband migrates within ten minutes of paddling to form a rolled cord across your hip. That cord, soaked and gritty with sand, then sits between two layers of neoprene compression and abrades in one place for two hours. That is not chafing in the mild sense. That is a raw stripe that stings in the shower for three days.
None of this is a criticism of cotton as a fabric. A cotton rib brief is a genuinely excellent everyday garment The Fabric Bible: What Your Briefs Are Actually Made Of. It is the wrong tool here, in the way a wool jumper is the wrong tool for a shower.
The chafing points, and where they actually are
Three places, and the same three on almost everybody.
The neck. By a distance the worst. The collar has to seal, which means it has to be tight, and every time you look up, which for a surfer is constantly, the neoprene edge saws across the back and sides of your neck. Add salt crystals and stubble and you get the classic red band that identifies a surfer at a wedding.
The underarm. Where a suit is thinnest and most flexed, and where a rash vest seam ends up if you have chosen your layer badly. Repeated shoulder rotation through thousands of strokes does the rest.
The inner thigh and the groin. Where two neoprene surfaces compress against each other, where any garment underneath is likely to have a hem, and where sand gets in.
The lesson is counterintuitive: adding a layer is not automatically protective. A garment with a seam sitting exactly on a rub point makes things worse, which is why triathlon coaches keep telling people that whatever goes under should be comfortable and free of anything that digs in, because digging in leads directly to rubbing between the layers.
Flat seams or no seams. That is the rule for anything worn under neoprene Underwear for Big Thighs: The Chafing Problem Solved.
Lubricant at the neck, which is not optional
For the neck specifically the solution is not a garment. Wetsuit lubricant, applied generously around the whole collar before you zip up, and on wrists and ankles if they rub. Triathlon guidance treats this as basic kit rather than a luxury.
Two practical notes. Use a purpose-made wetsuit lubricant or a plain body glide product rather than petroleum jelly, which degrades neoprene and will shorten the life of an expensive suit. And apply it to dry skin, before you get wet. Trying to grease a wet neck on a windy beach, having already fought your way into the suit, is a farce.
If you are still being sawed at the neck with lubricant on, the collar is too big and the suit is moving. That is a fit problem, not a lubricant problem.
The triathlon case: tri shorts, because you have to ride afterwards
Triathlon is the one discipline where the answer is genuinely different, because the swim is not the point.
You come out of the water and get straight onto a bike. Whatever you swam in is what you are riding in, and then running in, because you are not going to stand in transition getting changed. So the garment underneath is not chosen for swimming at all. It is chosen for the ninety kilometres afterwards.
Hence tri shorts or a tri suit. The pad in tri shorts is thin, dense and quick draining, quite unlike a road cycling chamois, precisely because it has to be swum in without turning into a sponge and then ridden on without being useless.
The one-piece tri suit is what most racers converge on: it goes on before the wetsuit, dries fast, carries the pad for the bike and is comfortable enough to run in. Whether you wear anything under the tri shorts is settled among people who race. You do not. Underwear under a padded short puts seams where the pad is meant to be and carries water into the bike leg What to Wear Under Cycling Shorts.
Check the rules for your event, because national federations do have clothing requirements and they are not always what you assume.
The surfing case: a swim brief, or nothing
Surfing has no second discipline, so the calculation is simpler: the minimum that satisfies the design brief. Two answers work.
Nothing is what a great many experienced surfers do, and it is entirely rational. No absorbent layer, no seams, nothing to break the seal, nothing to wring out afterwards. The objections are hygiene in a shared suit, which we will get to, and changing on a beach, which a decent robe solves.
A swim brief is the better answer for most people, particularly in a hire suit. Tightly knitted polyamide and elastane, it holds almost no water, dries in minutes, has no leg to bunch behind the knee of the suit, and stays where it is put The Sea Swimmer's Case for the Swim Brief swim brief.
That last point is underestimated. Getting in and out of a wetsuit involves a great deal of dragging fabric up your legs, and a garment with legs comes up with the suit and ends up somewhere ridiculous. A brief does not Brief, Jammer or Shorts: What to Actually Wear for Lap Swimming.
What does not work is board shorts, a surprisingly common instinct. They are built to shed water eventually rather than never take it on, they have a lace-up fly sitting directly under compression, and the hem lands mid-thigh where the neoprene flexes most. The cotton problem with extra bulk The Best Swim Briefs You Can Actually Buy.
Rash vests, thermal tops and when a layer earns its place
A rash vest under a wetsuit does one useful thing and one useless one.
Usefully, a smooth, seamless, quick-drying top gives your torso a low-friction surface against the neoprene, helping at the underarm and across the shoulders. It also makes the suit far easier to put on, because dry neoprene grips dry skin and slides over a slick synthetic. Anyone who has fought a 5/4 on a January morning will pay for that alone.
Uselessly, it adds essentially no warmth. It is a friction layer, not an insulation layer, and people who buy one expecting to be warmer are disappointed.
If warmth is what you want, the garment is a thermal swim base layer: a fleece-lined top designed to go under a suit, which does add real insulation. Triathlon guidance points people towards these once water drops below the high teens, alongside neoprene socks, gloves and a hood, which collectively matter more than any torso layer because you lose heat fastest from the extremities Right Support, Right Sport: What to Wear for What You Do thermal swim base layer.
Whatever you choose must be long enough to stay tucked. A short vest riding up under a wetsuit creates a fabric ridge across your midriff, exactly the hard edge under compression this article keeps warning about.
Hire suits, and the case that settles the argument
In your own suit, worn only by you, "nothing" is a perfectly reasonable choice and plenty of people make it.
A hire suit is a different object. It has been worn by a stream of strangers, many of them beginners who were in it for two hours and sweating hard throughout. Rentals get rinsed between uses at a good school and get a proper wash less often than you would like, because a busy centre turns kit around the same afternoon.
The interior of a wetsuit is warm, damp, poorly ventilated and full of organic material. It is one of the more agreeable places on earth to be a bacterium. Fungal skin infections and folliculitis are recognised hazards of shared watersports kit, and the standard advice from hire operators is straightforwardly to wear something underneath.
So: swim brief in a hire suit. Always. A decision that removes an entire category of problem and costs you nothing, because a swim brief was already the right answer.
For your own suit, rinse it in fresh water after every use, inside and out, and hang it over a wide bar rather than a wire hanger, which stretches the shoulders permanently. Salt, sunlight and chlorine age neoprene and swimwear the same way Chlorine, Salt and Sun: How to Make a Swim Brief Last.
What to do, by activity
Surfing, own suit. A swim brief, or nothing. Lubricant at the neck. No cotton, no board shorts.
Surfing, hire suit. A swim brief, without exception.
Open water swimming. A swim brief, plus a thermal base layer, neoprene socks and a hood as the water drops Your First Speedo: The Complete Buying Guide.
Triathlon. A tri suit or tri shorts, nothing underneath, chosen for the bike leg rather than the swim. Check your federation's rules before race day.
Cold water, any of the above. Extremities first. A hood, gloves and boots do more than any torso layer.
The argument
This question gets answered badly so consistently because people treat it as a modesty question when it is an engineering one.
A wetsuit is a sealed thermal system with a deliberately small volume of water inside it. Every decision about what goes underneath should be judged on whether it helps that system or fights it. Cotton fights it. Board shorts fight it. A tightly knitted swim brief with flat seams is invisible to the system, which is exactly what you want.
Which is, incidentally, why the swim brief refuses to die despite decades of English-speaking embarrassment about it. In genuine water sports it is simply the correct garment The Speedo Comeback Is Real (and We Have the Numbers).
Under a wetsuit, nobody can see it anyway. Which rather removes the only objection anyone ever had.
Questions people ask
What should you wear under a wetsuit?
A swim brief for surfing and open water swimming, or tri shorts or a tri suit for triathlon, where you have to cycle and run afterwards. The principle is that a wetsuit works by sealing a thin layer of water against your skin, so anything absorbent underneath holds cold water against you and works against the suit. Whatever you wear should hold almost no water, dry fast and have flat seams or none, so it does not create a rub point under compression.
Can you wear normal underwear under a wetsuit?
You can, but it is the worst available option. Cotton absorbs a great deal of water and gives it up slowly, so it sits cold against your skin for the whole session while the suit is trying to keep a small warm film in place. Saturated cotton also sags and rolls into a cord at the waistband, which then abrades in one spot under neoprene compression. Use a swim brief instead: it holds almost no water and stays put.
Does a rash vest keep you warmer under a wetsuit?
Barely. A standard rash vest is a friction layer rather than an insulation layer: it reduces rubbing at the underarms and shoulders and makes the suit far easier to pull on, but it adds almost no warmth. If you want to be warmer, use a fleece-lined thermal swim base layer designed for wearing under a suit, and add neoprene socks, gloves and a hood, which do more for cold water comfort than anything you put on your torso.
How do I stop a wetsuit rubbing my neck?
Apply wetsuit lubricant generously around the whole collar before you put the suit on, while your skin is still dry, and use a purpose-made product rather than petroleum jelly, which degrades neoprene. Do the wrists and ankles too if they rub. If you still get sawed at the neck with lubricant on, the collar is too loose and the suit is moving on you, which is a fit problem no lubricant will solve.