In the ocean near the north side of the map, you can check underwater to find a crashed airplane. When the airplane crashed, a briefcase full of money (around $12,000) also found its way to the sea bed. You can swim to find the wreckage and the briefcase. After you collect that briefcase, you can then immediately switch to another character and then switch back to the one who was located near the briefcase.
When you do, the briefcase will have reappeared. The character who is underwater will collect the case again and his supply of air will return. You can repeatedly use this trick to quickly amass a small fortune.