@2kwik4u Did you find the source of the problem?
I have not found a definite solution. I have some suspicions though.
Had the boat out for another 3 or 4 visits to the water since I posted. This past Sunday we went a bout 20 miles up river (and 20 miles back). The bilge ran once on that trip, but the water was "warm" not hot. As if it had just been sitting in the boat for awhile, or there was additional water in the bilge that hot water mixed with. It was definitely warmer than the river water, but wasn't HOT like it just came out of the engine cooling loop. The majority of that 20 miles though was spent ~6,500rpm or so, not at WOT.
Saturday I had the boat out doing watersports with a group of teenagers. I had some bilge action then as well, but again it was just luke warm it wasn't hot. That involved a lot of short WOT bursts to get the riders up, then settling down in the 6,200rpm range for the pull. Probably repeated this 50 times in 2hrs.
My suspicion at this point is two fold. The first is the inability to get the bilge completely dry. The second is a leaky rear cleanout tray.
When I first made this post, the boat had sat in the garage for 3 weeks straight. Any water that was in the bilge would have heated up significantly sitting in there. Our garage isn't insulated, has terrible ventilation, and the metal door faces west, so it gets the evening sun. 3 sides are bricked on the outside as well, so all the bricks heat leaks into the garage (it's a winter project to improve that insulation issue). I suspect I had water in the bilge that had heated up significantly, and then was rushed to the back during the outing. I'm not 100% convinced of this, as the water in my first post as HOT, like hotter than comes out of my faucet at home hot. My lack of recent problems has me thinking it's not a cooling system leak though.
I also noticed on Saturday while checking the bilge, that I had some residual water in the cleanout tray area, and it was leaking right through the screw holes. When I "sealed" the rear tray area earlier this year I didn't remove the screws, or the access cover, I just ran a bead of silicone all the way around the edges. Looked nice, and stopped a good portion of the water intrusion. I see now that I need to remove each screw, and seal under them as well. I might just pull the access cover and start over again from scratch on this honestly.
Overall it doesn't appear the engine is in any immediate danger. I've put another 10-12hrs on it since this first event, and I've not had a single problem with the engine. Even working it hard pulling 3 large teenage boys on a tube didn't seem to make a difference. I continue to get good flow from the outlet on the side of the boat, indicating I have good cooling water flow through the engine, and I had no hard starting or rough idle that usually accompanies a "vapor lock" condition from running it hard, then shutting it off and attempting a restart in short procession. The bilge isn't' running a lot when it does run. Maybe 10-15 seconds. Just long enough that I can see it, but not long enough to move back and get my hand in the flow to feel the temperature.
It bugs me to leave the source unknown, but at the same time it doesn't seem to be a problem to anyone/anything but me. Very frustrating honestly.