When you see a taxi drive past you, you can hail it by pressing , then enter it by walking up to it and holding down . Once inside, you choose your destination, and you are taken there. You have the option to skip the trip by pressing (much faster). For this achievement, pick up a taxi on one island, choose a destination on another, and sit there for the entire ride.
The shortest way to do this is to go to the map and set a way point (hit ) on end of a bridge. Get into a taxi, and have him take you there. Then get out, and immediately press again and get back in the cab. Now go into the map and set a way point for the other side of the bridge. Have him drive there, and the achievement will unlock.
You can also get this achievement on the bridge over to the other island. When the taxi goes to pay the toll, shoot at the car in front of you. The cabbie will get out, and the achievement should unlock.
**Can be done after completing story mode**