Well, I usually think of open sights as being the only kind of much use on a boat gun, so in that case the rail is prob'ly not much use at sea, though the boat gun surely comes ashore at times where it finds entirely different applications.
I'm not too familiar with red dot sights, never really seen one, but it seems maybe they would be of some use at sea, since they are advertised for close quarters use.
The red dot sights are GREAT for quick, close in work. They have a wide angle of view, and allow for both-eyes-open shooting. A dot or reticle is reflected onto your eye, representing a point out at optical infinity that is adjusted to match the strike of the round.
Not only do you not have to line up a front and rear sight (ghost ring rear sights are good iron sights for your purpose, too), you don't even need a good shooting position. As long as your eyeball can see the dot, the round will hit whater the dot is resting on, even if you are holding your weapon sideways.