, and can be done as soon as you're in the game proper after escaping prison.
Steal any standard car / van that has a boot (trunk).
Now go and run over a civilian / get out and beat them unconscious.
When they're down, go over to the body and press
to drag it to the boot (trunk) of your vehicle. Press the button again to stow it inside.
Now bring up the map and find the closest body of water - for me it was the river the runs east to west in the top left corner of the map.
Drive the vehicle straight into the water - don't worry, you can hit
to get out and swim to shore!
The achievement will unlock when the vehicle with the body hits the water.