Great question. And this is where it gets fascinating. Fish don’t sunbathe. Most fish live deep enough that UV barely reaches them. But the creatures that do hang near the surface—like coral, baby fish, and plankton—have evolved some wild tricks. Some produce natural sunscreen compounds called mycosporine-like amino acids (MAA). Sounds fancy, right? It’s basically nature’s SPF 50, built right into their skin.
Meanwhile, you and I have to slather on the stuff from a bottle. The ocean is pretty metal when you think about it: it lets light through, but only if you can handle the heat—or the UV.
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What about cloudy water or swimming pools?
Here’s where it gets real-world useful. Clear water lets UV through like a window. But murky water—think rivers, lakes, or that pond with algae—has particles and dissolved stuff that scatter and absorb UV much faster. If you’re swimming in a brown river, you’re getting way less UV than in a crystal-clear sea. Same goes for swimming pools: chlorine doesn’t block UV, but if the water is a little cloudy, you get a tiny bit of protection. Not enough to skip sunscreen, though.
And let’s talk about snow and ice. This is a fun curveball. Snow and ice are actually terrible at blocking UV. In fact, fresh snow reflects up to 80–90% of UV rays right back at you. So if you’re skiing or hiking on a glacier, you’re getting UV from above and below. That’s why mountaineers sometimes get sunburned under their chin.
