Like John said. Inspect that, and everything else closely.

Keep your antenna up. You need to find the problem before you start making assumptions or fixes. Even if you get new cylinder heads, and use a better gasket, you still need to find out exactly what is/was going on, IMO

The evidence is pretty convincing that water/coolant was getting into the combustion chamber, but if the source/cause is not obvious, I'm kinda wondering where all the cylinder pressure went. My very limited experience with small crack/water in the combustion chamber did not present the kind of cylinder pressure drop that you experianced.

Keep looking. Be sure to pull that piston as well.