Well, there are two different issues here with the exhaust under water. Whether you 'stall' due to water injestion, or whether you shut it off. If you stall out with the exhaust under water, or shut it off, you are going to be ok as long as the water level is below the height of the exhaust manifolds. If the engine is submerged, and it shuts down, the water will fill up the exhaust and any open exhaust valves, therefore any open cylinders. If this happens, dont try to restart it! Get pulled out, pull the plugs, drain the oil, refill the oil, crank the motor over to get as much water out as you can, then install the plugs and fire it up. You should be ok. If you ingest water into the air intake, and it stalls, thats a different issue! If you are stuck, and the manifold/ headers are still above water but the exhaust is under, you can shut it off and restart it as often as you like without issues. The water cant get into the cylinders. Now there is the possibility of a small amount of suction as things cool etc, but it all depends on how close the water level is to the exhaust openings in the head.
There is some experience talking here... its a long story hehe /forums/images/graemlins/smile.gif
Mike