Oh, and I'll also mention something else since my last campaign was similar in some ways to your idea (it was a Warhammer FRP version of Pirates of the Caribbean):
You may want to try and figure out, possibly in colaboration with your players, how much of the combat you want to be a ship to ship affair and how much you want to focus on the boarding action part. I developed a set of rules for ship to ship battles (these were partly an offshoot of earlier rules I'd developed for my Sky Galleons of Mars games) that tried to make those battles fun and fast and simple. Most battles tended to be a few shots fired at while the ships close and then shifted over into melee combat on the battlemat.
If your players are more used to hand to hand fighting then you need to come up with ways to either get right to that or else make the ship to ship part involve as much of the group as possible. I did that by giving each player a role during the battles. One handled all the Sailing rolls for handling the ship, another did all the shooting of the guns, another kept track of damage to the ship and crew and the fourth was the captain and actually decided where the ship would go on the battlemat.
If that's not really your players bag then you can either simply narrate what happens between the ships or reduce it to a small group of die rolls that indicate how quickly the ships close to boarding distance and what damage, if any, was sustained by either in the process.
