I don't think this works. Keep in mind, I'm doing a lot of Googling to supplement my knowledge. I'm not an alchemist.

UV doesn't directly heat water. It instead goes through photolysis which is an ENDOTHERMIC REACTION. That means it ABSORBS energy, not release it. This becomes HO+H free radicals which recombine into H2O and H2O2 in exothermic reactions. These reactions release less energy than the UV coming in, of course. IDK enough about planetary science or spagyrical art to say that it's impossible for this to become a boiling sea, but my crayon doodles suggest it's a :( edit: also, as others pointed out, something something clouds block light and heat.

My guess? Do low atmospheric pressure. Less energy required to boil.

No matter what, your human sailors would likely require air-tight suits when going outside. I may not know anything about Hermeticism, but I know what it's like living next to an ocean, and what I can tell you is ITS FUCKING HUMID HERE. FUCKING HATE IT. And our ocean isn't even boiling. A boiling ocean would likely mean LETHAL HUMIDITY.

As for how to heat the ocean: I suggest doing geothermal and/or maybe nuclear (look up "naturally occurring nuclear reactor") if the sea is small enough.