I purchased a 1987 Catalina 34 in late 2013 and the AC unit is located under the port side bench seat in the main saloon. Apparently this is where the holding tank was originally located. A bladder type holding tank was added in the wet locker under the nav table.
I replaced the 12,000 btu unit with a 16,000 btu unit and it has done a good job keeping the boat cool through the hottest summer days which are in the mid-90s. The issue with water cooled reverse cycle HVAC units is that they will not cool efficiently if the water temperature approaches or exceeds 90 degrees, and they will not warm efficiently if the water temperature approaches or drops below 45 degrees.
My boat is on the coast of Georgia so the summer and winter water temperature falls within the range required for the HVAC unit to function properly.