Ask anyone what their favourite parts of Assassin's Creed IV were and if they don't tell you they were the parts where you're on a giant ship then they're lying and are probably a terrible person.
Naval Action is all about tall ships with big guns, letting you sail the seas blowing splinters into the French, Americans, British, Spanish, whatever.
Taking place in an open sandbox world, it's going to be released in three stages. First up will be multiplayer combat. Next will be more involved combat, with crews, boarding and ship upgrades. The final stage will be the full and proper game which, like Sid Meier's Pirates!, will let you sail around conquering things and dabbling in more RPG-like pursuits.
On the surface it's all very serious, with promises of accurate ship and physics modelling, though this isn't a strict simulation, as they want to make it more exciting than just sitting around waiting for the wind to change.
Naval Action is currently up on Steam Greenlight.