The two worst things that have happened to any of my guns out hunting had nothing to do with a boat.
Shooting carts are sometimes not kind to guns, either. I bought and installed a couple gun scabbards on mine, over 15 years ago, that are not seen often. They hold the gun very secure, but it can be removed and replaced quickly. It could be adapted to a boat very easily, and mounted horizontally.
https://www.scabbard.com 
There is a strap that you can't see, that is tucked down inside at the top, that loops around the barrels to hold it in securely, but I don't use it unless I'm going some distance, as the design of the scabbard holds the gun very securely.
