Water in oil - 340 with 8.1L

I am hoping for some advice on the next steps to take. At the end of last year, the engine overheated and the head gaskets failed. Replaced the head gaskets, new oil and filters and put new exhaust manifolds on (see this link, post 22 here: http://www.clubsearay.com/index.php?threads/8-1-low-oil-pressure.110903/page-2#post-1363938 for more details).
Ran the engine for 8 hours over the winter since the repairs then changed and sampled the oil, and here is the report from Blackstone:

If is seawater, then the likely culprit would be the oil cooler. but isn't the oil in the cooler at a higher pressure than seawater? Am I missing another source of seawater?
If it is coolant, then either a gasket failed, or we missed a crack in the head, or the block is not happy.
What is the best way to narrow this down?

 

@PlayDate :Thanks for the list. I sent the two oil samples in at the same time, but Blackstone received them on two different days; I expect to hear back from them tomorrow and will re-post
@JHornsby3 : No I did not add any ATF to flush. In hindsight, yes I should have. I already have the correct fresh oil in it now; I will pull the oil cooler to pressure test it, which will loose some oil. I should be able to add ATF to top off.
@Scott215: For this round, I stuck with exactly what was in the Mercruiser manual. But will likely pick some up for the eventual next round.

 

@PlayDate: Below is the sample for the Port side, which is much better and backs up Blackstone's average value data.
As a side note, I have never had so few hours in a season, due to battling through this.

 

@PlayDate : Thanks for looking into this. This is an update from last night. So I pulled the oil cooler to have it pressure tested. Then I looked up. I saw corrosion at the top of the exhaust manifold. The manifold has seen 8 hours of run time. Here is what I saw.

The bolts riser-elbow bolts were not as tight as they should have been on this side (all the other risers still looked perfect). When I tightened the two outside bolts, more water weeped from the joint.
So then I pulled the elbow. More water weeped as I pulled it. Here is what I saw on the bottom of the elbow:

And the top of the manifold was very wet:

More corrosion on the inside than I would expect on 8 hours. On the right hand side of the picture, I saw moisture at the bottom of the manifold. I wrapped a socket extension with a shop towel, and pulled as much of the moisture as I could. It tasted very salty.
I pulled the plugs on that side. They look terrible.

It was getting dark and late, so before leaving, I hit the open cylinders with fogging oil. I will head back out to the boat later today.

 

No water came out of any of the cylinders when I pulled the plugs.

So when I started the engines last week, this engine sounded rough for a few seconds then sounded fine as it heated up.

So the question is whether this is the cause or a symptom of something else?
If I see signs of this much water on the outside of the manifold, is it likely that exhaust air is also leaking at this joint, which at idle may not create enough lift to get the raw water all the way up the elbow so it falls back to the manifold? Any other conclusions to come to?

It seems that taking the oil cooler to an exhaust shop is not a good use of time, since the oil in the cooler was perfect. I am thinking I need to reinstall the oil cooler, pull the rest of the plugs, put the exhaust back together, and turn the engine over asap (treat the event as if I am trying to avoid a hydrolock).

 

Again, thanks for the comments.
STBD SN = 0W393939
No coolant at the gasket or the manifold. Raw water only.
I will check the elbow again. It is stainless and appeared to be in good shape prior to reinstalling the heads. And there were no signs of salt on the outside of the elbow (that is one good part of phantom black paint), like there is on the top of the manifold.
Below is a diagram of the closed cooling system.

Let me know what you think of the leak at the elbow manifold gasket; it is possibly leaking exhaust gas as well as water. Exhaust pressure is what lifts the raw water up through the elbow. If there is a leak at that joint (especially at idle), there would be less pressure to lift the raw water. It could be that there is a standing column of water sitting on the turbulator, and when the engine is turned off, the water falls down on the manifold. When the engine is restarted, some water is let into the engine. The water can't compress, so it leaks by the cylinders and into the oil.

The failed clamp was on the seawater at the outlet of the air actuator (Item 2), before it elbows up to the transmission cooler. (Item 3). The raw water was being dumped into the bilge. Without the raw water, the exhaust got hot very quickly and the coolant got hot as well.

 

I agree with you, @scofflaw.
Here is a picture of the same engine as mine, but for an outdrive application. The raw water and exhaust are the same as mine: I believe the raw water is pumped up through the bottom of the exhaust manifold (at the fitting with red threadlock), out the two rubber hoses at the top of the manifold, and into the stainless elbow where it mixes with exhaust.