1/ Don't fall in. (I'm serious. It's really easy when looking through a lens to take a step or two to frame better. And then one mpore step or two and you're soaking wet.)
2/ Watch the exposure. As you zoom in and out, you'll get more or less sky into the shots, and this will probably change the exp[osure setting on the camera. So, althogh the light hitting the boats does not change, your exposures wil be lighter and darker. The way to beat this is to use manual exposure.
3/ Long lenses mean tripods. A 300 mm lens is long on a regular camera, on a Rebel, it's really long. Monopods, or leaning on a fence, work well, too.
4/ Polarizer if there are big fluffy clouds.
5/ Pay attention to backlight. With sailboats if is often a good thing. With dragon boats, you might want to move.
BAK