Re: A Neptune has risen from the weeds to live again.

[Casey Lynn]

Yep. Exhaust back pressure from static water level can definitely have the effect you are experiencing and you are possibly on the right track.
Something you have not mentioned however is piston skirt to cylinder wall clearance.
You can have great top compression (compression above the rings) and not have enough crankcase compression to run the engine in the water.
Check what you have mentioned but keep this in the back of your mind if you find the others are okay.
Tapered cylinders, worn piston skirt, previous mud dauber damage, etc.

PS….I can’t believe you took on that project! Good on you I guess??