My only suggestion is a goodly length of parachute cord tied to the wrist of the gun and the other end fastened to an oarlock. Less expensive in the long run than multiple salt water dives later.

Wait a minute, I got another one-Mossberg 500. Actually cheap enough to be disposable, and will eat any ammunition you feed it.

I guess I wouldn't worry too much about an 870, but, I'm thinking the SW isn't going to fare as well, in any event. Good Luck.
Best,
Ted