Hi Joseph-
I am also a sailor and have gone through the same analysis process in the past. Unfortunately, some hard choices have to be made. My conclusions are as follows.
The typical tactical case is a 50 ft steel hull coming out straight at you 30 to 50 nm away from a coast (Malaysia, Columbia, Vietnam, you call it). Once you have established hostile intentions, you need 2 weapons - a light, high volume of fire one to keep the vessel away and the people off the decks and their firing positions, and a heavy weapon to attempt to destroy the seaworthiness of the vessel. For the first one you could go AK-47 or Mini-14 stainless. For the second one you need a .50 BMG. Don't forget to shoot at the top of the roll only.
A second tactical situation is individual defense when on the coast, at anchor for instance. For that I recommend handgun, tactical shotgun, and civilian looking rifle. For shotguns, the Mossberg 590 marine is OK (stainless), although the problem is that there are many returns on this firearm - lousy quality. I finally opted for a Benelli M3 Super 90, which also enables you to shoot big flares (convertible from semi auto to pump). For a handgun, I would recommend a Beretta Inox or a Glock. For a civvy looking rifle, I would recommend a Mini 14 stainless or a hunting rifle w/ as much stainless as possible.
Be careful - many countries simply forbid you to bring any weapons whatsever - such as Mexico, where you WILL end up in jail for 10 years +, or Thailand, which will execute you - even if you declare your firearms. What this means is that choosing to take firearms along means you will have to avoid all the countries for which it is a significant problem.
Take care - I logged many sailing hours on a Merit 25 in the 80s, and have several friends who singlehanded one to Hawai.
Mckysdad