You'll have to earn this while boarding enemy ships. As your crew pulls the ships closer together, you'll be able to board the enemy's ship using ropes that you can swing from. There's one rope right next to you at the wheel, another one at the bow of the ship, and a third one accessible by climbing the mast. Run towards any of these ropes with  and you'll grab it and start swinging.

To make the assassination count for the trophy, you have to press  to assassinate someone while you're still on the rope. If you jump off the rope and then press the button, it won't work.

Assassinate five enemies in this way and the trophy is yours.