Not sure of what boat model you have but all of the Grady's I have owned use a salt water. My Sailfish has 2 seacocks for the head. One an inlet for salt water and one an outlet for discharge. both of these seacocks are located under the hatch right outside the cabin door. Check to see if the seacock is open. One time I had a clog and it turned an eel had gotten through a dent in the hull screen and it became lodged in the hose.
Fixed it...it was clogged on the RAW water inlet. I repetitively filled the bowl with fresh clean water and went to fill position (not flush. After about three cycles, it cleared.