Which Assassin's Creed game introduced naval exploration as its central gameplay feature?

Assassin's Creed IV: Black Flag introduced naval exploration as its central gameplay feature.

Released in 2013, Black Flag follows Edward Kenway, a Welsh privateer who becomes involved with the Assassins and Templars during the Golden Age of Piracy. Players sail across a large Caribbean world, visiting islands, ports, and underwater locations.

Naval combat had already appeared in Assassin's Creed III, but Black Flag made sailing, ship upgrades, sea battles, and maritime exploration the defining structure of the game. Edward's ship is the Jackdaw, which can be improved with weapons, armor, and cargo capacity.

Assassin's Creed Rogue and Odyssey also feature substantial naval systems, but Black Flag is the series entry most specifically built around pirate-era seafaring. Its combination of stealth missions and open-water exploration became one of the franchise's most recognizable formulas.