Which Assassin's Creed game introduced naval warfare as a major gameplay system?

Assassin's Creed III introduced naval warfare as a major gameplay system.

Released in 2012, Assassin's Creed III added substantial ship-based missions in the North Atlantic and along the American coast. Players control the Aquila, a sailing vessel commanded by protagonist Ratonhnhaké:ton, also known as Connor, and engage enemy ships using cannons, swivel guns, and maneuvering tactics.

Naval missions were integrated into the main campaign rather than appearing only as a small optional feature. The system required managing wind, positioning, ship movement, and weapon timing. It became popular enough to inspire a much larger naval focus in Assassin's Creed IV: Black Flag.

Black Flag is often remembered as the series' pirate and naval game, but its naval mechanics were introduced in major form one game earlier. Assassin's Creed III is therefore the specific answer when asking which title first established naval warfare as a substantial gameplay system.