If you don't need S resolution, you could always just get a 40D and one of the smaller aperture IS zooms that reach 300mm. That would give you more crop-free magnification, and cost a lot less than either of your other options. I assume a lot of boat races are conducted in bright light, where you could probably use smaller apertures than 2.8 to no ill effect. Something like the 70-300 DO IS might be fun, since it is far more compact than any 300 prime.
The downside would be a dimmer viewfinder, and less sealing protection from the water. The 40D combo would be so much cheaper that you could afford to replace two or three that were ruined by spray, and still be ahead.
But then again:
The 300f2.8 is insanely nice.