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 Rover backpack, you wouldn't give it a moment's thought. The camera compartment is touted as being fully waterproof, with the inner drypod protected by a patented zipper mechanism. And if you find yourself getting thirsty along the way, this pack has a separate segregated space for a hydration system -- no worries about mixing camera equipment and water in the same bag, something that's almost always a no-no. A spacious top compartment provides room for lots of personal items.
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.