Mattato- I dont see a flash from the boat being of any use. For one I doubt it would be powerful enough to reach and for two, most strobes arent capable of a higher flash sync than 1/250th. And in mid-day sun it is rather difficult to get the shutter speed to 1/250th or slower without over exposing. Now if you were in a tube near the rider you might have better luck... still be tricky.
My suggestion... just do a spot reading on the rider and shoot at that exposure. Or just shoot early or late in the day
(Thats also when you could have some fun with the strobes)