In a marine vessel (such as a yacht or other boat) in the range of 45-75 feet a chilled water air conditioning system is provided having a significant advantage over split central systems. Utilizing two-four water chilling modular units the chilled water capacity only needs to accommodate about 75-90% of the calculated BTU heat load for the vessel, while the air handlers are rated at about 100% of the calculated BTU heat load. Each modular unit has no refrigerant connection exterior of the casing containing all of its components, including a condenser coil, evaporator coil, compressor, reversing valve, and expansion tubing, all operatively connected to each other by refrigerant lines within the casing. Chilled water is circulated into and out of the evaporator coil by an exterior pump and expansion tank operatively connected to each of the modular units, the chilled water passing through a coil unit of an air handler. An exterior seawater pump pumps substantially ambient seawater into the condenser coil and is subsequently discharged to the exterior of the marine vessel, through its hull.