The Y-Tube is to return the hot water that comes from the heater core back to the engine
OK, makes more sense. I was thinking the Y-tube was cold water supply, not warm water return. Thanks for the clarification.
My other boat has a shower, uses cold lake water mixed with warm engine water in the right proportions to make a nicy comfy rinse. I suppose a heater doesn't need any mixing valve, just the Lo-Med-Hi fan speed.