Join Date: Jun 2006
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Your idea is more suitable for heat, as in piping water from the block into a car's heater core, which is blown from a fan and into the cabin. The water then proceeds back to the mixing elbow. That's a cheap way to capture otherwise wasted heat energy, and is the same idea as a heat exchanger plumbed into a hot water tank.
A marine air conditioner (I have a Marine Air 12,000 BTU condenser/heat pump unit) can take sea water of most temperatures and use electricity to either provide cool air or warm. I've used mine at dock with great success, but with a 10 amp draw on AC power, I wouldn't use it through an inverter unless someone was dying of heatstroke. If it wasn't already installed, I wouldn't have all that conduit through the boat either, but I'm OK for space and it is a blessing at dock. Offshore, we will use a diesel heater and dorades/windscoops for heating and cooling.
For smaller boats, it's either not feasible due to the space, or due to the fact it can't be easily run once off a dock. Books like "The Warm Dry Boat" go into this stuff extensively, and how critical airflow (both passive and active) is to a comfortable, mildew-free boat.