While I have a lot of friends that use GoPro's, I am a big fan of the Contour cams... www.contour.com they make some models that are inherently waterproof, mine I had to buy a waterproof case for about $30.
I have the Contour GPS, and that actually tracks speed and elevation. Not as important in water sports, but I use it on my sportbike, my M3, and mounted to my helmet while snowboarding. The profile of the contour makes it WAY better as a helmet cam. I see people with gopro's attatched to their helmet and I can't help but think of that "whack-a-mole" game. If I had a cam sticking up off my helmet like that, it'd be gone with my first close call with a tree branch or crash on the slopes.
Now, gopro's are great for being waterproof as well as they have an amazing ability to filter wind noise. However, while Contours aren't as good at filtering wind noise they do have better overall sound quality to their videos. They also don't have that wide angle fish eye look that the go-pro's have. they don't have a viewfinder, but have lasers that are used for aiming. Mine also has an app where you can use your smartphone as a viewfinder to see what it's going to look like when you record as well as the ability to change settings.
Most my videos are car/motorcycle/snowboard, so I don't really have anything boating to contribute yet.