shooting from the boat all you need to worry about is spray when the boat is turning or perhaps hitting wave. personally, i always put my camera inside a clear plastic bag when hooting in the wet. depending on your lens, you may be able to elastic the bag to the lens hood. that makes it virtually waterproof. you probably do this already but short bursts and fast shutter speeds will give you your best chance to get some keepers. i would try setting the focus manually to rope length minus a couple of feet and letting the DOF cover the skier. this way the AF won't try to keep up when you drift off the body.