I apologize if my details weren't enough in the beginning. Yes I took the factory motor out and built a mark IV 454 block up to a 496. The mark IV blocks are the strongest blocks Chevy ever produced to handle the power I was trying to obtain. I also did not want the headache of fuel injection.
So, in a nut shell, the boat has no power loss up to the 5400 rpms the I run. Now, when I did the compression test I did it cold, not at operating temperature because I was hoping to find low readings. As far as winterizing the boat, I am the only one to touch my boat. I drain all the water out of the motor first, then I use a kids swimming pool with a pump to circulate the antifreeze through the motor while it gets up to operating temp. Then I drain everything out and leave it open all winter. As you can see I am very thorough on that for it's not worth losing a motor. The oil cooler is not factory mercruiser either, I have an extra large oil cooler. Problem is, I have to pull the motor to get it off. I need to try and pressure test it in the boat. It makes sense now that you have all pointed it out. I never thought of it.

My motor build was a copy of the old HP-i motor from mercruiser if any of you remember that one.

Bypass the water to the oil cooler and fire it up on the hose and look for oil if pressure checking is too difficult.

How is the cooler plumbed on the raw water side? If similar to Merc to where it is second inline after the power steering pump cooler could you not just remove the hose feeding either the circulating pump or crossover and dump the water into a container and look for a telltale oil sheen? If it is introducing that much oil into the raw water stream you would think it would show up in the water left in that hose or on the inside of the hose itself. Just a thought if it is that much of a beech to try and pressure test the cooler with the engine in the boat.

I have thought of just draining it and looking at the water through some Mason jars.

Pulled the oil cooler.... put 30 psi in it and it has held for over an hour... looks like a leak down test is in order on the motor.

Being this is a few years old, Im hoping you got this resolved. What did you find your answer to be? Having the same issue.. cooler tested okay.