You can also point the camera down at the water, taking the sun and sky out of the frame, lock your exposure and then reframe and focus on the rider. As another poster noted, some of the background will be blown out, but that is how you'd pull color out of the rider.
When I shoot my kid just inside a bright window, I do the same thing. I think that's how most window portraits are shot....
(Not my photo -- got it from azildalions.ca)
(Message edited by supraman on November 10, 2009)