How about Ortlieb roll-top bags?
I think you could achieve the same waterproof-ness, plus better ability to attach to your rack if you buy a pair of Ortlieb roll-top panniers. I've got the standard type (with a backpack-type lid), so it isn't submersion proof, but I with a roll top closure, I bet it would be just about as watertight as a dry bag (and would also work in your boat).
Russ