in our 1985 Catalina 30, all the drains have a water level just below the bottom of the sink. The sinks are just above the water line of the boat. The drains don't empty very fast but will eventually. If your boat is similar, it will never drain as fast as your house drains do.
If you are in an area where barnacles or mussels grow, make sure your thru-hull and lines are clear. If the thru-hull is shared as a drain and inlet, make sure you have not pulled debris into it. In the fall we sometimes get pine needles, leaves, etc. pulled into our inlets.