It's not so much the amount of time submerged as it is the time since it resurfaced. It's the oxygen in the atmosphere that causes rust and other types of corrosion. I worked on a salvage ship and we recovered a Bering Sea crab boat that was submerged for a year. As soon as it was refloated, the engineers drained the main engine (a Cat D398 as I recall) crankcase, filled it with water-soluable oil, drained the cylinders and rolled it over. They changed the oil, with more of the same and did it again. Within a few hours from resurfacing the engine was running again, but just for a while - until we could tow the boat back to port for engine teardown.