Which game was the first Assassin's Creed title to feature naval exploration as a major gameplay system?

Assassin's Creed IV: Black Flag was the first Assassin's Creed title to make naval exploration a major gameplay system.

Released in 2013, Black Flag expanded the naval missions introduced in Assassin's Creed III into a central part of the game. Edward Kenway sails the Jackdaw through a large Caribbean world containing islands, settlements, wrecks, forts, and other ships.

Players can freely explore much of the sea, engage merchant and military vessels, board targets, and gather resources for ship upgrades. This design made the ocean more than a sequence of separate missions; it became a second open-world space alongside the land-based environments.

Assassin's Creed III did include important naval missions, so it is often cited as the origin of the series' ship combat. The distinction is that Black Flag was the first entry to make naval travel and exploration a dominant, continuous system. Later games such as Rogue, Origins, and Odyssey developed related ideas in different ways.