 If it's scarred or worn with sand and so on from the water, it may not be smooth enough to seal against water leakage. Look for a shrink card at the hardware store with a rubber flapper valve and circular metal valve seat in combination. You simply glue the new valve seat ring onto the old hole with a mastic included in the package. Get your questions answered live Saturday afternoons on KRDO Newsradio, 105.5 FM, 1240 AM, and 92.5 FM.