If you end-up pressure testing, as David Marchand suggestion offered, then I'd also look at the peripheral connections and devices.

Connecting fan coil heaters and piping to your cooling system is a neat idea, but it's not without its exposure to risk. I would certainly put a gate valve to "lock-off" and isolate such connections. In some cases, engines won't be reaching or maintaining operational temperatures because you have added such "oversized" radiators of heat. This isn't an issue when running continuously in moderate temperatures and engine loading -- but I'd be cautious about it cold temperatures if you're just using your engine to charge batteries and provide a little heat in a cold anchorage.

Offshore a few years ago, I noticed that the pressure valve on our hot water heater had opened-up while we were motoring for a long time at higher RPM's. The hot water heater's protection from becoming a pressure vessel gave me a clue that the heat exchanger was coated inside and was not functioning correctly. But that's a long story, I've told before, and won't burden you with now.