I have a
hurricane, which is also hydronic (and which I would recommend). You can have a
heat exchanger to get heat from the
motor, but it is optional. The heater's
coolant loop is completely independent from the engine's
coolant and does include it's own
pump and header tank.
You'd never want to connect the two because you want to protect the integrity of the
engine cooling system, and because the extra volume of coolant through the whole boat wouldn't
work for the engine
cooling system.