Which Assassin's Creed game was the first in the series to feature a fully navigable open-world ocean?

Assassin's Creed IV: Black Flag was the first Assassin's Creed game to feature a fully navigable open-world ocean.

Released in 2013, Black Flag made Caribbean sailing central to exploration rather than treating ships as a limited sequence or occasional transport. Edward Kenway commands the Jackdaw, upgrades it, and uses it to travel between islands, settlements, wrecks, and other maritime locations.

Assassin's Creed III had already introduced notable naval missions, so the two games are sometimes confused on this point. Black Flag expanded that earlier idea into a large, continuously explorable maritime environment with ship battles, boarding actions, underwater sites, and ocean wildlife.

The ocean system became one of the game's defining features and strongly influenced later series entries. Assassin's Creed Rogue, released the following year, also emphasized ship exploration, but Black Flag established the fully open-ended naval format first.