Quite a few, I'd imagine. There is one by the docks in GTA III, there are loads littering the sea in Vice City, including a car grave just beside a bridge. Near Faustins home in GTA IV, there is a road with cones infront of a broken railing. On the rocks below there are lots of burned out taxis. On the coast of Alderney, there are TONS of shipwrecks. hope this helps, even if exact figures aren't available to me.