So with a lot of people saying that they would buy a game with just the ship missions, it seems that the developers may have heard you. Announced today is Assassin's Creed 4. In this one the new assassin is a pirate. There is not a lot of details yet but there is expected to be more info released on March 4th.
http://www.eurogamer.net/articles/20...ameplay-on-ps3