For the PC port, "Grand Theft Auto 3" does not have a cheat code to make cars float on water. However, it is possible to fly over bodies of water with a flying car cheat code.
To activate the flying car cheat code in "Grand Theft Auto 3," enter the game, and then use the keyboard to type the code "chittychittybb." When the code is successfully entered, a notification saying "cheat successful" will appear in the upper left hand corner of the screen. To use the cheat, find a nearby vehicle and gain enough momentum to fly the car off the ground.