I found this pretty difficult. Climb over the wall behind the ships wheel and use the lift to take you up the mast. Turn towards the enemy ship and you will see the rope. Hold
and run towards the rope, let go while you are swinging and watch the top right corner of the screen until it gives the option for assasinate. Press
Managed in the end by going for enemies on a Man of War mostly or waiting for enemies to be at the back of smaller boats. Hope it helps.