Which Assassin's Creed game first made naval exploration a major part of its open world?

Assassin's Creed IV: Black Flag first made naval exploration a major part of its open world.

Black Flag expanded the naval gameplay introduced in Assassin's Creed III into one of the series' primary systems. Players sail the Jackdaw through a large Caribbean map, visit islands and ports, attack rival ships, hunt sea creatures, and upgrade their vessel. Much of the game can be approached from the deck of the ship or by swimming to shore.

Assassin's Creed III had already introduced playable ship missions, so Black Flag did not invent naval gameplay in the franchise. Its innovation was making sailing and piracy central to the overall campaign rather than presenting naval sequences as a smaller supporting feature.

Assassin's Creed Rogue later built on this approach with a ship called the Morrigan and an Arctic setting. Black Flag remains the entry most strongly associated with the series' open-world pirate sailing and Caribbean exploration.