Instructions
Progress through the game until you reach the "End of the Line" mission. Drive to East Los Santos and locate Big Smoke's hideout. Exit the car after the conversation between C.J. and Sweet. The game will suggest using a nearby SWAT Tank to gain access to the hideout.
Scale the fences and get near the SWAT Tank. You will see a gunfight occurring between the Ballas and the SWAT team. Stop at the intersection north of the battle area and pull out your sniper rifle. Clear out all the Ballas and SWAT members before approaching the tank.
Climb inside the tank and drive it to the import/export crane in San Fierro. Exit the tank and board the crane. Use the crane's magnet to grab the tank.
Move the crane as close to the water as you can. Swing the crane arm, hover the SWAT Tank over the water and submerge it. Wait until you see the "Mission Failed" message on the screen. Pull the tank out of the water and return it to dry land. Remove the magnet and get back inside the SWAT Tank. Drive it to one of your garages and enjoy your new toy.