User blog comment:JAlbor/Assassin's Creed: Black Flag Q&A Opportunity/@comment-6517-20130816112629/@comment-1888015-20130816212038

You wouldn't be able to explore Africa or anywhere else not intended for the game because Ubisoft wouldn't have a whole world map on their game engine.

You'd be more likely to just keep sailing along a blank stretch of sea until you get to the very edge of the game world, and if there wasn't anything holding it back, you'd sail off the perimeter of the map and fall endlessly, until you reset your game.