In an interview with Eurogamer, Game director Ashraf Ismail said that to sail across the open water of Assassin's Creed 4: Black Flag would take about 30 minutes. "We don't talk about it in terms of size, that would be a little bit unfair - there is a lot of ocean in the Caribbean. But it is the biggest game we've ever made," Ismail said.
Ismail says that the main narrative of the game will last about 15-20 hours but "we're hoping that people [get] lost in the Caribbean world, in the toybox we've created." Large swathes of the map will be open ocean but Ubisoft has stressed that you won't have to sail everywhere as there will fast travel system and side missions available while sailing, so it won't be bored.
While you're out sailing players can always stop and harpoon some Great White Sharks, whose resources can be used to craft items. While there will be other available sea creatures to hunt we've yet to see them, including the possible hunting of whales.
Assassin's Creed: Black Flag drops on October 29 on Xbox 360, PlayStation 3, Wii U and next-gen consoles later this year.