There are two main ropes on the Jackdaw from which you can swing from ship-to-ship on: one in the back (nearest you) and one in the front. Make sure you have your hidden blades equipped and when you are closest to the ship, hold
, swing, tap
, and with a bit of luck you'll perform an assassination as your first kill. This method is easiest done on smaller ships with easier enemies, like sentries, on them, ie schooners.
There are also two ropes higher up on the Jackdaw, giving you more height to get to enemies on larger ships more easily, but I've always found the closest ropes to be the easiest.