Since you asked for the best lens and not what you could get by with, then the Canon 70-200mm IS is the best to shoot from the boat with. The 2.8 may not be required for bright sunny days but if you shoot late evenings or other sports, you will wish you had it. It is very pricey though. I use it exclusively when shooting from the boat. Get used to focusing manually as you will have better results in my opinion. If you are shooting from the shore, then a 300mm or 400mm on a mono pod may be necessary to reach the action.
Here is a link to some of my recent work:
http://plsphotography.smugmug.com/Sports-2009/Wakeboard-2009 And from last year including shooting from shore with a 400mm 2.8:
http://plsphotography.smugmug.com/Sports/Wakeboarding-2008