Let's get the obvious out of the way first: Paul Henreid is no Errol Flynn. Oh, he tries hard and grins a lot, but he doesn't have the same panache. When it comes to buckling a swash, Flynn can't be topped. But that doesn't mean The Spanish Main isn't a lot of fun, because it is.
Henreid is Laruent van Horn, a Dutchman who's turned pirate because he's been wronged by the corrupt Spanish governor (Walter Slezak) and thrown in prison. He escapes, and turns pirate. Now he's known as The Barracuda, and he's the terror of the Spanish Main. When he captures a ship on which the beautiful Francesca (Maureen O'Hara) is sailing, he's also going after the escort ship. She begs him not to and marries him to prevent it, although she's on her way to marry the corrupt governor, whom she's never seen.
You can probably figure out the rest. O'Hara and Henreid are really in love, though they won't admit it (that would be too easy). There are betrayals and imprisonments and sword fights, and all ends just as you'd want it to.
The Technicolor makes the picture wonderful to look at, as does Maureen O'Hara. Walter Slezak steals every scene he's in by being both comic and menacing at the same time. The only reason he doesn't steal the whole picture is that Binnie Barnes, as Anne Bonney, does a good job of stealing all her scenes, too. She's terrific.
One of my reasons for watching this movie was that Fritz Leiber (the father of the renowned SF writer of the same name) has a role as a priest. You can see him in the crummy clip (there's no trailer available) provided below.
The movie's a lot of fun, and the writers are obviously channeling Sabatini. Not up to the Flynn pirate movies, but well worth a look.