Water in Oil and Cylinders

Hope someone can help. I have an 85 SN 2001, 351PCM engine. The boat has always run perfectly and started easily but one day last Summer, I suddenly ended up with water in the cylinders which made the boat first hard and then impossible to start due to hydraulicing. Checked the oil and found water there also. This all seemed to happen overnight. The engine always ran at 160 degrees and definately did not overheat.
I pulled it apart and checked everything out, pressure tested exhaust risers and heads and found nothing obviously wrong except for one corroded exhaust valve seat. Head gaskets looked fine. Did a complete valve job and reassembled, ran on land but not in the lake yet.
The boat stays in the water tied to the dock all summer and I'm wondering if heavy wave action could have forced water up the exhaust? There are rubber flaps but they would tend to open, I think, on the downstroke of the waves. This also happened to my last boat (350 Mercruiser) which makes me suspicious. Has anyone seen this before? I can dock the boat with the bow out but that is a pain.
Appreciate the help.

My friend had this exact issue with his 73 Century Coronado, water entering thru the exhaust and then directly into the engine/cylinders. With his, however, there is very little rise from the transom to the manifolds. I'm sure Pete is correct that it's not likely this could happen with your modern setup/risers.

I suspect the rusty exhaust valve seat is a symptom of the water intrusion, not the cause. Water should not be entering the cylinders in the first place.

When you tore it down, did you note any signs of where the water was coming from? Any water in the runners of the exhaust manifolds? Any rusty water tracks in the valley that would indicate an intake manifold leak? Which exhaust valve was it? How did you pressure test the exhaust manifolds? Presumably the heads were magnafluxed when you had the valve job done?

Im not entirely clear- what is the current state? Freshly assembled and run on the hose with no issues yet? Or are you seeing more signs of water intrusion since the reassembly? You may still want to do a compression test- a failure would most likely indicate a cracked block.

I highly doubt water is coming back in through the exhaust ports. A few people have seen the engine turn backwards a revolution or 2 upon shut down, which can be enough to suck water back into the engine and hydrolock it (I havent heard of any credible reason for why this happens though).

SKI NH, Water in Oil and cylinders. That's very scary to hear as I just pulled off the top of the exhaust manifolds on both the port and starboard sides. Plugs 1,4,and 8 showed signs of water contact. The Starboard side manifold show signs of rust, where as the port didn't, but cylinder 8 still had water contact. Of course the oil was nice and milky. Any of this sound familiar to anyone? I, like everybody doesn't want to pull off everything to find out its simply the exhaust manifolds need work. Help!

Dadof8 wrote: SKI NH, Water in Oil and cylinders. That's very scary to hear as I just pulled off the top of the exhaust manifolds on both the port and starboard sides. Plugs 1,4,and 8 showed signs of water contact. The Starboard side manifold show signs of rust, where as the port didn't, but cylinder 8 still had water contact. Of course the oil was nice and milky. Any of this sound familiar to anyone? I, like everybody doesn't want to pull off everything to find out its simply the exhaust manifolds need work. Help!

Not scary at all. Fairly common problem on here. Need more info on how you found the water in the oil. What the conditions were. How it was running at the time. Did it overheat? Has it ever overheated? Does it run in salt water or has it ever run in salt? How many hours on the engine? What maintenance has been done? What engine is it?

Most common problem is exhaust manifolds either cracked or riser gasket shot. Is there any signs of water tracks on the outside of the exhaust manifolds? You pulled off the top of the exhaust manifolds. Why? Standard gasket replacement or troubleshooting the water intrusion.
Other potential problems are cracked head, cracked block or blown head gasket. Have you done a compression check or leak down test?

More specific info would help out a lot.

By the way, welcome to the site. I think you'll find a lot of helpful info here. Pics and more info about your boat are always welcome too.

I suspect the backfire would have been the result of water in the cylinders not the cause of it.
You've taken care of any of the potential issues that could have caused your problem without confirming which one was the problem to start with.

You said the mechanic tested the manifolds. Was that before or after the gaskets were changed?

If it doesn't have any issue now, run it and be well. If it happens again, some more investigation would confirm the problem before tearing it completely apart.