when i was in costa rica, the guys shooting the rafting had rebels and some maybe had the T1 it was moving too quickly. The company either positioned them or the photographers positioned themselves near where the hardest rapids were, ie: best shots cause the boats were either going to get thrashed and/or people would look like they were battling the hardest rapids.
But i saw no need for high FPS for what they were shooting, as the boats were staggered enough that it was adequate time for the photographer to snap 2-3 shots per boat. Personally, id want an autofocus because it looked kind of boring shooting the same thing over and over and if it was paid, id want it easy. Maybe you would enjoy it more doing manual, i dunno.
this is an example of one their shots. Nothing special. D40 or d40x would suffice ($125-200 body) + a lens you feel comfortable with. d3100 is pretty nice for what it is. The people buying these photos don't seem to know or care as much as any photographer on here would, so i wouldnt sweat the equipment TOO much