Look for the brain box. Many boats that have multiple AC units, have an electronic box that turns the water pump on. Basically, generally each AC unit is on it's own breaker and all the breakers may not be on the same power leg. So the box can receive power from any and all AC units to turn the water pump on. If you only have one AC unit on the water pump, the brain box isn't required. You likely have a brain box with the three AC units, just reuse it for the two on the same pump.
Ted