4-108m cooling problem

I'm in Neah bay right now looking for suggestions. My "new model" engine (combined heat exchanger/tank/manifold) overheats in about 10min from a loss of coolant. No detectable external leaks, no coolant in oil. My thoughts include head gasket or exchanger leak. Either might result in coolant in exhaust, but it's hard to detect amidst the sea water. Any other possibilities?

More info:
Coolant tank holds pressure until engine cools completely
Engine struggled to turn over one time.

This suggests head gasket to me (pressure would bleed off faster with exchanger leak and struggle suggests a hydraulic lock in cylinder). Rather it wasn't head gasket!

G'day,
As well as cooler seals and head it could also be the exhaust manifold they have a habit of leaking at the rear where the mixer connects. After next run remove the injectors once engine has cooled and header tank has emptied crank engine over water will spurt from the crook cylinder although this could be coming from a corroded manifold so when removing check carefully for a water trail etc. Borrow a radiator presssure tester to check cooler seals.Remove exhaust hose or mixer to check manifold end. Do which is easiest for you first.

Another odd symptom is that even after an overnight cooling, the coolant is still holding some pressure...

G'day,
This could mean that the affected cylinder has stopped at tdc not allowing the displacement area to open up or you may be dealing with a heat [thermal] affected crack or joint.

I removed the Bowman end caps and found no evidence of fluid leakage. Since I'm not a real mechanic, and don't have all the tools or time I would need, I called in the locally recommended mechanic and he's going to dive in to the assumed head gasket next week. We agreed I'd get a better job with the work done at his shop, so the engine is coming out as well. I did nurse the boat to Port Angeles, so at least there were mechanics around...