The Assassin's Creed franchise is taking the extremely well-done sea-faring segments of AC3 and fleshing them out into it's own full game. We don't know for sure, yet, but based on this map (the reverse side of a pre-order poster)-
we can infer that the game will span over the Caribbean and be open-world between several islands. We'll have both sea and land adventures, and possibly even explore sunken ships on the ocean floor. In that short sequence where we see Kenway swimming towards the wreckage underwater, you can even see a shark in the background! And in this screenshot, a whale!
Releases on October 29th on PC, 360, PS3, WiiU, PS4, and "Next-gen consoles," (the next, unannounced XBOX).