I saw a recent show with Bear trying to take a raft across a lake or river with freezing water. It was just a plain old dumb move. His rafter was too flimsy, it couldn't keep him from just being flat out wet, and there was no way he was ever getting across before hypothermia had it's way with him.
In all fairness Les has built rafts too. The one in the Georgia swamp could barely hold him above water and the one he built in his Alaska special had to be abandoned half way through his crossing. He had to swim some pretty icy water to get back to shore.
In the latter case however he had medical teams and experts on hand but it was still risky.