If you haven't done it already, check cylinder heads and intake manifold torque. I ran into this same thing once, no leak until boost.

What head gaskets are on it? can you tell? if someone put cometics on it and the surface was not prepped right they can leak under boost only, so hard to find water leaks because we cant pressure test coolant system too easy...

What kind of water pickups do you have? Kinda sounds like it is water pressure related.

No idea on head gaskets

Running the same water pickups that were on the boat when new.

https://scontent-iad3-1.xx.fbcdn.net...55&oe=55B45B9F

See if there is little rivots sticking out under heads you will be able to see them if cometic

Had some milk on the valve covers on my 525sc. Pulled the top end of the motor off to check everything. The intake gasket water passages were leaking into the lifter valley. Pulled the exhaust, and heads, checked everything. New head, intake and exhaust gaskets. Been fine since.

leaning towards an intake gasket and or the intake itself especially around the water passage area could be slightly eroded away.

UNice looking boat man. What HP and what's the top end.

That's the case on my buddies HP500's in his Slingshot. Eroded the intake