You can get this just after you've got to the top of the agency tower - you'll need to have at least three stars for agility for that, preferably four stars. That itself gets you an achievement.
The High Flyer achievement in Crackdown worth 14 pointsMake your way to the top of the Agency Tower
Make sure you are on the highest of the three towers, and stand on the highest wall. And jump! Keep your joypad pushed towards the nearest water and you should land in it easily.