Sunburn
If you've never spent much time on the water, then you probably don't realize just how fast you can get sunburned. That is because you are getting a double dose of the sun - both direct and reflected off the water's surface. You can get burned very fast.
The best protection against the sun is a big broad-brimmed hat (not a baseball cap - that is nearly useless.) If you periodically dunk the hat in the water or wet it from a bottle, it will stay nice and cool.
If it's not too hot, consider a long-sleeved shirt as well. Sunblock is likely to get splashed-off. If the sun is very strong, try to stay in the shade, which is often surprisingly doable.
I've never met anyone who takes this advice. Enjoy your sunburn.