Having circumnavigated in the 70's, voyaging to some pretty "dangerous" places like the Sudan and SE Asia, I always felt comfortable with a 30.30 carbine and a double barrel 12 gauge w/ 00 buckshot for defense.
However, with the new breed of "pirate" attacking sailing boats with evil intent, those weapons seem of little value against a handful of aggressors armed w/ AK's in a very fast, maneuverable craft.
As a professional captain, I am very familiar with the laws regarding firearms where I've traveled. Quite simply; "war weapons" including the ancient M1 carbine amazingly, are a definitely NOT acceptable ANYWHERE. Pistols are frowned on by most countries and if reported, may need to be held ashore.
Sporting weapons, on the other hand are not a huge problem anywhere.
After many hours of web-searching, numerous emails to chemists (I was considering some sort of napalm like defense) I really had not found anything I thought would be of real value.
Last evening I received an email from a friend with this link; Dragons Breath in action
This one MIGHT do the trick! Though they may be illegal in some places, with the shells secreted in a box of buckshot or whatever; they are pretty easy to hide, and they require only a single shot 12 gauge (I doubt that you'd need more than one shot if you delivered one of these on target). I'm impressed!