Somewhere about 300 yards from where you put your canoe in to the salt marsh for the sea bird photo tour it'll hit you: um, what happens if this thing turns wrong side up? You've got a lot of gear that probably wouldn't take well to meeting the ocean close-up.
If you had this amazing LowePro DryZone backpack, you wouldn't give it a moment's thought. Even fully loaded, it floats in the water. It is touted as being fully waterproof, with the inner drypod protected by a patented zipper mechanism. And if you find yourself in less precarious circumstances, you can just close the inner zipper and the top clip so you'll have easier access when you need it.
The harness is custom-designed for comfort and wearability, and a rubber handle at the top makes it easy to grab and go when you're moving fast.