I would have thought that conventional air con on a boat is not very practical due to the heavy power that it consumes - generally well over 1kW. Also very expensive. We have been using an evaporative cooler that uses about 1% of that power. It has to be filled with water regularly for the cooling effect to work. If the water runs out then it just works as a fan. We have found that it does reduce the temperature by a few degrees and our dog seems to like it. She often lies down on front of it to get the best cooling effect. Cost was a fraction of the cost for full air con and the power demand is so low that we can leave it running overnight even when cruising.