Bear Grylls seriously thinks that floating a log raft down an unknown, unscouted high-gradient river is the quickest way out of high mountain wilderness?! That man is dangerous to anyone who listens to him.

I think a wetsuit with splash gear would be sufficient for 40 degree water on a roadside run with an experienced guide and a proper raft. I've swum in 40 degree whitewater with a 3mm wetsuit and a drytop, and while it was cold, a good wetsuit would be sufficient for a relatively short swim.

On the other hand, drypants to match my drytop are one of the best purchases I've made! For any cold run where you may be in the water for any substantial duration, dry gear is a must.