Thoughts on water in 350

I finally got around to tearing down the 350 (260 Merc) that came in my '82 Renken. The fellow I got it from knew nothing except that the engine was locked up. When I first pulled it out of the boat a couple of years ago I drained several gallons of clear water out of the oil pan and then put it over in a corner of the garage. What I found on teardown was the intake manifold full of rust, the 2 cylinders with partially open intake valves full of rust, the crankshaft majorly scored at the rear main bearing and the cooling passages of the block crammed full of rust. So, the only good things left are the cam and the rods. I was planning on a new motor anyway so that's not a big deal but I am curious as to how the water got in so I don't recreate the same situation with the new motor. I'm thinking most likely the whole boat was swamped or is there some other way for the intake to get filled up with water right under the carb?

 

What is condition of the exhaust manifolds and riser? Usually on a stock motor water in the block is either caused by poor winterizing or water entering thru the exhaust due to corrosion.

 

Manifolds and risers are not rusty or cracked as far as I can tell. I redneck pressure tested them with a garden hose, some fittings and hose clamps and didn't see any leaks. It's the intake manifold that's packed full of rust right under the carb. It had a flame arrestor on it and the engine cover in place when I got it, but that doesn't mean someone hadn't left it off at some point.

 

Take a look at exhaust bellows. New?

Chances are it was the source of water in engine you describe.

 

Isn't there a water passage from the head to the intake manifold right next to #2? A breach in this would flood both the intake manifold and cylinder.

 

If only the two cylinders with open valves were rusty, chances are the water got in when the motor was just sitting.