Can I sue for my medical costs after a rafting accident?

I can't think of ANY white water rafting place that WOULDN'T have you sign a liability waiver. Since you were still going on rapids then they have no fault. Now had something been wrong with the raft itself and it sunk after you started going in then you may have something that the waiver wouldn't cover. Since the injury occurred doing what you intended to do(maybe you didn't intend to go over such rapids, but you intended to go over rapids) it is covered. The other thing I question was was anyone else injured? No one else being hurt would excuse them ever more.