Camel,
My solution was a fannypack, which is about the only time I use one. I also made my canoe gun a Taurus 85CH, which is a stainless steel revolver.
That way in case I spazz on the water, it's easy to deal with from a firearms standpoint.
I don't recommend a shoulder holster for two reasons. One, the rig will probably foul up your paddle stroke. Two, if you're wearing a life jacket, good luck trying to accommodate a shoulder holster as well.
Another option would be a thwart bag. That's basically a fanny pack that straps onto the cross-members normally found just behind the bow but in front of the bow seat, another at the center of the canoe, and then a third just in front of the bow seat. Normally I keep sun tan lotion, bug spray, camera, etc. in my thwart bag, but you could store a firearm there. Or just strap your fanny pack to a thwart.