There's the makings of something here: a pirate comedy or a swashbuckling drama, but, ultimately it's neither one or the other enough to make it a good movie, which is rather a shame. Had it actually been a "Trinity Boys Do Pirating" sort of movie, it would have worked. It such a case, it needn't have had a compelling plot: just let Terence Hill and Bud Spencer humorously beat up other pirates and corrupt sailors while having their own dumb misfortunes and bad luck. The movie already had the two comedians and plenty of other very capable actors and actresses, including the beautiful Silvia Monti, Monica Randall, and Diana Lorys.
Alas... Bud Spencer is barely even in this movie and Terence Hill ("Blackie the Pirate") is rarely funny cast as a more traditional leading man type. In fact, the sort of stuff you expect of a "Trinity" movie, Bud's impossible strength and toughness, Terence's agile and offbeat fighting style are farmed out more to the supporting actors, which is actually one of the better parts of the film: Blackie's crew al seem to have their own special talents: one, for example, has excellent aim with a canon or pistols, other is very strong, etc. A crew of pirates, each with their own specialties but only in certain situations starts to almost write itself.
Well... whatever. It's not that movie. Watch it if you'll watch anything By Hill or Spencer or watch it if you'll watch any kind of pirate movie, just don't watch it if you're expecting Terence and Bud to be doing a buddy-buddy pirate comedy.