The ships that you seize, by boarding, can be enlisted in Kenway's fleet, on condition that your computer, or gaming console, has a direct connection with Ubisoft's servers (you are notified about that by the highlighted connection icon, right next to the minimap). It is worth mentioning that, in the case of Xbox 360, you do not need the Xbox Live gold membership. Using this functionality, however is not possible in the offline mode, the system must be hooked up to the internet.
When protagonist Edward Kenway boards a ship, he has the option to add the ship to his fleet. This fleet functions similarly to your assassin recruits in previous titles; you can send them on missions, and after a varying number of real-time minutes, the ships return with money and spoils. However, we've confirmed that your access to this whole loop is gated by the Uplay Passport. In other words, if you borrow the game from a friend who has already redeemed the code (or buy the game used), you don't get the fleet and the benefits that come with it in your single-player game.
On one hand, the fleet does include online elements. You are able to have your friends assist you to make missions go faster, for instance. On the other hand, there's nothing about it that demands this assistance; once you can use your fleet, you can do the missions solo. Previous installments in the series, like Assassin's Creed: Brotherhood, included similar offline versions of this mechanic.
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