Heat Exchanger Built-In
As far as I know, most commercially available marine hot water heaters have a built-in heat exchanger. My '81 C-30 has raw water cooling with no heat exchanger. The engine water is routed through the water heater, making hot water via the water heater's integral heat exchanger. I also know of propane flash heaters that heat the water on demand as you use it. I'm sure similar models are available in Diesel.
