Parachute directly from the top of the Galata Tower to the golden horn.
The Galata Tower is the huge tower next to the Assassins' HQ. You can fast travel there using the Tunnel Entrances scattered around. There is actually a Memoir Page on the top of the Galata Tower.. If you don't have a parachute already, purchase some from any tailor. Climb to the top of the Galata Tower and face South (based on map). The golden horn is basically the water that separates the two bodies of land on the map. This is where the ferries are. Just jump and hold , heading south. You will slowly glide to the water. Once you drop in, a few second later the achievement will unlock.