How long was it sitting? Well, I'm crossing my fingers as I can't get back to my boat until tomorrow. Maybe just maybe the existing oil with serve as a protectant. You guys have me freaked now!!! And I knew better....

Update. I pulled off the manifold today and the exhaust chambers all had very minor surface rust. Nothing to worry about so I'm happy to say that I caught it all in time. I hosed everything down seriously with WD40.

Problem is, now I'm stilling trying to identify root cause. I'm having all kinds of bad luck with this boat. Got a leakdown tester today and sucker turned out flawed. Ticked off. Likely, I'll be rebuiding it entirely but maybe, just maybe I'll have a cracked exhaust manifold.

Guys, I need help theorizing here. So here's what I've done so far.

1) Pulled plugs...passenger side of motor had water in it.
2) Fogged motor to prevent rust & corrosion.
3) Pull off risers. Passenger side exhaust manifold had a ton of water in it. Drivers side a-ok (plugs were dry too).
4) Removed passenger side exhaust manifold. Dryed it out. Plugged water inlet passage and filled with water updside down. No sign whatsoever that water is creeping through the wall into the exhuast port. Anything else I should do to detect a cracked manifold?
5) Compression checked all eight cylinders. #1 thru #8 all ready 165-170 psi. Seems perfect.

What now? Motor appears fine (cross your fingers). I'm going to suck up some oil to see how it appears.

Right now, I was just going to replace the manifold gasket and both riser gaskets. And plumb everything back up & fire up. I'm very afraid though that I'm missing something & might get water into the cylinder.......bend rod! But I can't seem to find anything wrong with her.

The entire event sequenced around a full speed reverse into a massive yacht's wave (yes, dumb as hell & whole reason your friends shouldn't drive). My theory is that it created a massive pressure wave which forced some water all the way up the exhaust into the manifold.....getting some into the heads. All I can imagine at this point. It was easily 100 gallons that capsized over the rear of the boat soaking all 4 rear passengers from head to foot. So not you normal wave.

Thoughts? Other areas to check out prior to firing up? Thought I'd fire it up.....shut it down quickly & pull plugs to check for water?

Are your exhaust flappers in place?

I just had about the same issue... turned out to be a bad riser, not the manifold, that was leaking the water. If you fire it back up and once again find water in the cyclinders, pull the riser off the manifold, depending on the type, if you have more water in the two middle cyclinders- risers leaking.

Could you actually detect a crack in your riser? Or just narrow it down to the riser, replace & back to good?