[Gerry Zagorski]

Sounds to me like what someone mentioned above... Your risers are likely shot allowing water to leak down and through the exhaust valves into your cylinders causing hydra lock.

If this is the case you are lucky you caught in time since in many cases this could lead to a catastrophic engine failure and you have to replace the whole engine.

First thing I would do if I were you is to get that water out of the cylinders by cranking the engine with the spark plugs out, especially if its saltwater. Next thing is to squirt some oil into the cylinder heads and continue to crank the engine to coat and protect them from rusting and then put the plugs back in . After doing this I'd take the risers off, have them inspected and if they are shot have them replaced. Risers are usually good for 5 to 7 years in salt water, even if you have a fresh water cooling system. Most people replace them every 5 to 7 years as preventative maintenance because if they fail while you are underway, you could ruin your engine.

Once you are done with everything you now want to make sure and change your oil before you start it up since there could be water in it. In fact, just to be on the safe side, I might change the oil a 2nd time if I see any traces of water in the oil after the first change.

Hope you dodged a bullet here and all it is is the risers.