I have a 12000btu mermaid model on my 380. It heats as well as cools. There is a spare slot on the A/C power panel for the wiring.
I installed the unit as you suggest under the aft part of the port setee. The control box is in that area, as well. I draw the intake air from the forward section of the settee and discharge the cool air directly out the side of the settee – towards the nav station. The grill has deflectors so I can move the cold air upwards and to both sides.
I have a number of Hella fans that I use to move the cool air around the boat. I put the intake thru hull in the same compartment as the shower pump and the strainer and water pump in the bilge area adjacent to the thru hull. The outlet is on the port