Which Assassin's Creed game introduced naval battles as a major recurring gameplay feature?

Assassin's Creed III introduced naval battles as a major recurring gameplay feature. Players control the ship Aquila in missions involving naval combat, sailing, and battles along the Atlantic coast during the American Revolution.

The naval sequences became one of the game's most distinctive systems, using wind, maneuvering, broadside cannons, and swivel guns. Connor can return to the ship and take part in missions that expand beyond the game's land-based frontier and cities.

The feature was later expanded dramatically in Assassin's Creed IV: Black Flag, where sailing and piracy form the core of the experience. Assassin's Creed III was released in 2012.