4 men on an 18' cataraft, experienced boatman. In a riffle, they bumped a rock. The victim lost his balance and fell into the river. He was wearing a fastened life jacket, but it quickly slipped off. While the raft remained lodged on the submerged rock the victim floated quickly downstream. He was last seen swimming downstream instead of toward shore. He was face down when the raft caught up. CPR failed to revive the victim. Authorities determined that he probably died of heart failure, not drowning. The water was not unreasonably cold.