According to GameInformer, players will not able to have the option to add ships to their fleet and utilize them in single player campaign without an online pass.
This feature is similar to recruiting and commanding your assassins on missions to gain influence and money in Brotherhood. Black Flag's version has an online component that allows your friends to assist you in missions, so it is understandable why an online pass is required. However, it is also possible to use your fleet completely solo but you would still need the online pass regardless.
This only affects used games wherein a previous player has already redeemed the code for the online pass when bought brand new. If you buy the game used and have no intention to play multiplayer, you may have to purchase the pass separately to get the most out of the single player campaign.
Ubisoft has not yet commented on this story. We will update this once more information is available.
The game is now released for current generation consoles, while the next-generation versions will be released as a launch title. The PC version will be available starting November 19th.