This method will require you to have Ubisoft Club, which can be accessed directly in the main menu of the game. After completing episode 1, you should be able to redeem a reward called The Assassins (Female)
from Ubisoft Club.
Next, once you have ship customization unlocked after recruiting your first lieutenant as part of the story, you can choose a crew skin in the bottom right corner and choose The Assassins (Female)
to replace your crew with female assassins.
Now, set sail on the open seas and attack another ship unlock they have the "dock" symbol next to them. Instead of invading their ship, line them up, and hit them from the side while using ramming speed (Hold A). If you hit them at a high speed, their ship with be ripped in half and this will unlock.
NOTE: It will still unlock if you are playing as Alexios or if you have a male lieutenant. Only the extra crew members matter.
Doing this will also unlock: