People use “rash guard”, “swim shirt” and “sun shirt” for the same thing, but they were designed for different jobs. Here is the difference and which one you want for a day in the Keys.
Rash guard
Born in surf culture to stop board wax and sand chafing the chest. Tight, stretchy (nylon/spandex), often with a higher neck. Great in the water; clammy and warm on a boat, and the tight fit shows every hour of the sun.
Swim shirt
Marketing shorthand for a looser rash guard: usually UPF rated, still a swim-first fabric that holds water and dries slowly.
Sun shirt
Designed for time around water as much as in it. A loose, lightweight performance knit rated UPF 50+, moisture-wicking and quick-drying, cut to move air. It works as a rash guard for snorkeling (it will not chafe a vest), then dries on the ride back and looks like a shirt at lunch. That is what we make.
Which one for what
| You are… | Wear |
|---|---|
| Surfing, paddling on a board | Rash guard (tight fit matters) |
| Snorkeling, swimming, sandbar | Sun shirt — long sleeve |
| Fishing, boating, sailing all day | Sun shirt — long sleeve or hoodie |
| Beach with the kids, hot afternoons | Sun shirt — short sleeve + sunscreen on arms |
Fit
Sun shirts are meant to be loose. If you want a swim-fit for snorkeling, size down one; for the boat, take your usual size. Women’s crop tops are the exception — they run snug, so size up.