The issue: Push down the glow plug button, turn the key: Nada.

Mechanic replaced the starter. Worked for a couple of weeks, then turned the key: Nada.
Mechanic said there might be an internal leak that lets water into the bell housing which causes the starter to rust inside and seize up.
Mechanic replaced the head gasket, also replaced the new starter with the original starter. Engine started, ran great. Next day, turned the key: Nada
Mechanic said maybe the exhaust system wasn’t set up appropriately, added a 90 degree elbow hose higher up. Engine started, ran great. Next day, turned the key: Nada.
I got mad, smacked the starter with a rubber mallet. Engine started. But that was Thursday and I'll bet it won't start when I go there tomorrow.
I dread going to Dura Mater. It is not good to dread going to my boat.
Mechanic says he's stumped. Any advice from my knowledgeable singlehanded colleagues?