If you're going to carry a revolver, don't assume it's foolproof. The cylinder will freeze up in wet - especially salty - conditions in fairly short order. A revolver will not fire at all if the cylinder won't rotate. A semi-auto will also be finicky if not well maintained, but will give you one shot (assuming you keep one in the chamber). It's all personal preference, but I prefer the shotgun even in a boat ... you can control the penetration by what rounds you load in what order. Pistol grips, folding stocks, variety of barrels all give you more options depending on your intended uses. I taught my ex - just sit in the corner and point toward the door/hatchway. With non-slug rounds it's about impossible to miss. OTOH, all guns are going to require care and maintenance. What is the effective (accurate) range of a speargun anyway?
"You've come to the wrong shop for anarchy ..." ~ Master and Commander