I've used dry bags on many a canoe/rafting/kayaking trip and they've always worked wonderfully. Never put a DSLR in one, because I haven't been on the river since acquiring my D5100, but I've carried point-and-shoots, cell phones, etc and never had a problem with anything getting the least bit wet.
That said, not sure I'd take my DSLR on the river with me. Because I've seen people capsize, and the dry bag come loose and shoot off downstream before anyone could reach it and grab it.
Could be those people just didn't know how to tie a proper knot. I still wouldn't risk it, because stuff happens.