Quoting ZTFab:
"I had just taken off my logs and risers to have them ceramic coated before I started replumbing and I put new gaskets on between the risers and logs. The surfaces on the logs and risers seemed damn near perfect, in fact everything came apart really easy."
The riser/log gasket just might have done the trick. Looking at the design of the Harman Marine water log, it would seem that the riser/log gasket could possibly leak internally back into the exhaust, but NOT leak externally into the boat.
Quoting OLDSQUIRT
"Might just pull the manifold off the head but leave the water lines connected. Run water through the system and see if it is coming from inside the log/riser or from the engine."
Good stuff there!