Which Assassin's Creed game first introduced playable naval exploration as a major feature?

Assassin's Creed III first introduced playable naval exploration as a major feature in the series.

Connor can command the ship Aquila in optional naval missions and explore portions of the Atlantic coastline. These sequences use ship steering, wind, positioning, broadsides, and boarding actions, making naval combat more than a brief story diversion. The feature proved popular enough to become the foundation for the much larger seafaring systems in Assassin's Creed IV: Black Flag.

Black Flag is often credited with naval gameplay because it makes piracy and ocean travel central to the experience. However, Assassin's Creed III introduced the major playable naval system first. Earlier games included water travel and ships in limited ways, but they did not offer the same dedicated, controllable naval missions.