Use the highest shutter speed possible for the sharpest images, although depending on what lens your using you may not want to drop the fstop too low, as your dpeth of field may not capture the whole person in focus (ie face or chest may be in focus, but arms in front or behind may be out of focus). I would say pretty much ideal is f8 f11 or so with a shutter speed of 1/1000. Obviously you may be able to get away with some +/- on either side, and this may not always be possible with the light you are working with. I'm sure others will have some tips as well.
WRT to focussing when shooting from the boat i will often autofocus once on the rider and then turn the autofocus off, or switch it to another button on the camera so it is not re-focussing everyime you press the shutter button. The rider is always the same distance from the boat, so no need to switch focus.
Just my opinion. Good luck, hope to see some samples posted here!