Tim-
It sounds like you either don't have an antifreeze line/circuit  running from the engine to your water heater, or you do, and it's not working. I'm assuming you have a line. It's likely that if you trace the antifreeze lines, you will find a valve somewhere, and if it is closed, it will be preventing hot antifreeze from circulating in the water heater. Locate the antifreeze circulation pump on the engine and trace the lines from that. If uncertain about what is flowwing through a particular hose, loosen the hose clamp and see what drips out.

I had a 1986 C30 with a valve on the antifreeze line that could close so it wouldn't circulate through the water heater. I'm not sure why Catalina did this. The vavle was located in a really obscure place on the C30. It was a gate valve (like on your garden hose outlet). I couldn't suggest where to look on the C36.

Tim, there are several posts on the subject, but having had this happen this spring, my local 'ancient mariner' said that after sailing the boat a bit and 'burping' the system, the airlock in the line should clear.  While that fixed it for us, I'd go with Paul's suggestion first.  I added valves on either side of the hot water intake/output to allow me to shut the lines off and blow the water out in the fall.  A PO might have done the same.