I've seen a few guys lock them in, then place a pan head screw snug next to the locking mechanism & into the base, so that the lock slider can't move back into the free/release position.

I've lost one in the salt, and almsot lost one a month ago when a front seater didn't lock it, went to pop the rod out on a summer, and flipped the rod holder overboard. I jump over after it, went under, grabbed it and climbed back up over the side of the drifter, all while going downstream with my friend fighting the fish

I've also seen people paint the inside of the locking piece, so you can see with a quick glance around the boat if any paint is showing, indicating a rod holder that isn't locked...