I have a SS ranch rifle for exactly the uses you describe.
On land, I'd go AR, but for a boat gun, nothing beats the stainless steel mini. The AR will rust, and has more nooks and crannies for grit and fish scales. Get a SS/black Bultercreek folding stock on there, a couple boxes of tracers, and you're in business.
Accuracy on a boat is relative with motion. You should fire as the boat is moving up a wave, not on the down-side; i imagine that if a boat was doing something suspicious, i'd let go a tracer or two across their bow, and they'd probably opt to pirate elsewhere at that point. The other benefit of tracers on the sea is that a fuel tank is a bigger target than a torso.
/if only they made SS factory mags. I use pro-mags on the boat since they don't rust.