Thanks IGGY, that's really clear. So I have a system with sea water.
I'm now waiting for a new heater, to re-install everything. Only the heater was clogged, full of rust, as I have tried to start the engine just after having withdrawn it to see what would happen (the hoses coming from and toward the engine being free with nothing connected on it.) I saw the water was coming through , and decided to stop the engine immediately to avoid having too much water inside the boat !
I have read some people were connecting the 2 hoses together to have the water circulating inside of it. Is this the way you make sure no rust stays in the engine ?
Also when everything is fixed, I guess it means, when using the engine, I can have hot water without having to press the heater switch on, right ?