What viscosity oil?

what Im asking is the psi that you refer as normal before it starts to drop, what is the oil temp at this time ? big heavy boat ?

If you’re definitely full, you could try running a lower level. Might be whipping up a froth at high rpm.

between 50-60 psi
no idea about oil temp
it in a 38 sonic

2 weeks to go for boating season so
will take it out and have clearances checked to rule that out

When apart, Make sure any windage tray or rear baffle that is supposed to be there, is there. Also head oil drainbacks aren’t being blocked partially by anything. And pump screen to pan clearance.

I think you need to do more testing, rule some things out before just saying we're going to pull it out, so its a twin application but the other engine that was not rebuild is ok ? call the shop and get the specs on the bearing clearances, was the crank turned in the rebuild

Remove oil filter and cut open, if it looks like Christmas in the filter you have junk.
if clean filter, let her eat.
Good luck.

When apart, check oil pump pickup tube/foot for being cracked or loose at the oil pump. Could be sucking air when oil level is lower due to being up inside engine.

I would try to answer but the explanation of what is going on leaves my head spinning.