I had the same problem in my Searay Sundance 310. It is due to clogging of the small tubings in the air conditioning unit. Needs routine maintenance. This is what you need to do:
Purchase a groco flush cap that fit the strainer for the intake of the AC unit (the strainer is in the bilge; brings raw water into the pump). Close the intake raw level, remove the strainer cap and the filter and screw in the groco flush cap. It has a nipple that allows insertion of the end of a hose. Connect the hose to a water source and flush the system for a good 20 -30 minutes. If you wish, you can put a small clorox tablet inside the strainer; this will kill any barnacles that may be inside the air conditioning piping. After completing the flushing, remove the plastic flushing cap, reinstall the original one and you should be fine. Please make sure that when you reinstall the cap, there is no air inside the strainer as that will prevent the pump from drawing water.
You may need to do this once or twice during the seasson if you are docking in areas with high debris.
