I didn't even play the third one, just watched my roommate play it, and that was enough for me. For me, Assassins Creed falls into that Fable category of games where they're fun and they seem like a good idea, but halfway through you just drop your controller and find something better to do.

The pirate setting is cool, and more boat stuff might be interesting, but being a cynic I'm forced to wonder where all this is heading, now that the Assassins Creed series is no longer satisfied with simply historical periods, but has to take on specific character types. Will there be a cowboy, samurai, and ninja game in the near future? It seems like it. Which isn't necessarily bad, because those would all be cool places to visit, but since all my feelings toward the series are summed up with "Meh," I think I'll pass.