I agree on the go pro idea. Just need about 3 of them I think.
Any thoughts on how to pressurize the system when boat is on trailer

Check you exhaust hoses and clamps. Sometime they will leak at higher pressure/RPM's. Seen it more than one. Had the same symptoms on my own boat, chased that water in the bilge issue. Took a mirror and went around all the exhaust hoses and clamps. One was not even all the way around. loosened it up and moved it even, water leak gone. Rechecked a couple of weeks later and it was about 1 turn loose. .

My 2000 502mpis have high water pressure too. Higher than I like, but its been that way since I bought it 600ish hrs ago. With new impellers, they will peg the gauge at 35+ at WOT.

Ok will check all clamps again. You do agree the leak is from the starboard motor because of the fluctuating water pressure?

Also isn't the water in the exhaust no longer under pressure? Because the gauge is moving up and down I keep looking after the sea water pump but before the exhaust. Am I correct on this?

Bilge doubles as a hot tub , I don`t see what the issue is.

Not quite the answer I was looking for but funny anyways.

Crack in the block? I'd look for rust on the block also.

I need to get back in there and look again. When I changed the exhaust I did not see anything unusual. Also when I run it on the hose I get no water in the bilge but of course I can get the same extended amount of water pressure.

look at the freeze plugs while you're upside down