I originally bought my 4 inch Security 6 for kayaking and fishing halibut in Dutch Harbor. Caught 1 halibut that nearly rolled me over. But the Security 6 is a bit big for handling from a Kayak. One of those derringers might be the ticket.
Remember, bouncing around in a kayak and scrambling with a paddle, it could be very easy to get the barrel pointed in the wrong direction. BE CAREFUL. That was the lesson I learned in my kayak.
I'd put a lanyard on the gun.
Regarding a holster. I think ripstop nylon is usually way too light weight. Heavy cordura nylon would be the way to go - still without something to stiffen it, it will be floppy. Or, consider using some old neoprene waders and cut to a holster shape and glued and stitched. I'd consider making a belly or chest band of the neoprene on which to mount the gun.
I'd strongly suggest wearing some sort of floatation. I realized I didn't want a couple extra pounds of steel strapped to me - I don't float even without the extra weight.
Maybe make a neoprene holster and attach it to the kayak deck with industrial velcro.
Just some ideas. I too have been intrigued with the Bond guns but have never handled one. Seems like I read somewhere that they can be as heavy as some snub revolvers but with less shots. Maybe a stainless Charter 38 would be an answer and more versatile for other uses.