Self contained marine air conditioning units should be located in the main cabin to be air conditioned. If you want to use one air conditioning unit to cool more than one cabin you can do this using a splitter to divide the duct and give you an additional outlet for the second cabin.
Typical positions to install a marine AC unit would be under a settee, under a bunk, at the floor of a wardrobe or hanging locker or under the V berth.
You can install a marine AC unit in an engine room or lazarette, but it must be completely boxed in so the return air comes ONLY from the inside cabin or cabins of the vessel that you are air conditioning.
Marine air conditioning units must recirculate the air conditioned air of the boat, not hot/humid fresh air. The AC unit will draw in air from the compartment it is in (via the return grill) and discharge this air through the insulated duct to the outlet grill. Every pass of air over the evaporator will reduce the temperature by approx 10°C and will also reduce the moisture and humidity of the air. It may take several passes of air through the unit to reduce the ambient in a cabin from 30°C+ to 19°C with 50% humidity which is why it is important that you are not introducing new air from outside.
