You need to unlock a pirate fleet through either the Ubisoft Club Reward or through Hasbro Game Club. Both of these will require some form of being online. The Hasbro side seems to be cheaper, and then you can save your Ubisoft for a discount if you want it.
The quickest way to do this is to have a second controller and play a local PvP match. You can control the second players ship layout to hit the targets with the most efficency and then have the second player just choosing to skip every turn. You should be able to finish the match pretty quickly, rinse and repeat until you get the trophy to pop.
*Update*
I got far enough in the campaign where I was playing with a pirate fleet. If you just replay these missions you should be able to pop the trophy and won't need to get on to Ubisoft Club or Hasbro Game Club. Good luck!!