Couldn't it make sense if you only use the boat every day or two, and it's docked in the sun when not in use? I can see that it wouldn't make sense for cars since cars are typically parked (1) out of the sun, in covered/enclosed structures, and (2) near power outlets, which provide an alternate source of energy.

Some people might want to use a boat infrequently, and not near charging stations. But perhaps a speed boat isn't the type of boat someone would choose for this due to lack of sleeping quarters, fridge, etc.?