Which Assassin's Creed game introduced the series' naval battles and ship upgrades as a major feature?

Assassin's Creed III introduced naval battles and ship upgrades as a major feature of the series.

Released in 2012, Assassin's Creed III follows Ratonhnhaké:ton, also known as Connor, during the American Revolution. Its frontier and coastal environments allow players to pilot the warship Aquila in missions involving naval combat, navigation, and attacks on enemy vessels.

The naval system includes cannon fire, maneuvering, boarding encounters, and improvements to the Aquila. These mechanics became especially important in the following year's Assassin's Creed IV: Black Flag, which expanded sailing into a central open-world activity.

Earlier games included water travel and occasional ships, but Assassin's Creed III is generally credited with introducing the fully developed naval gameplay that Black Flag later made famous.