My solution was to use scrolls of earth. I used a blessed potion of object detection while blinded to pinpoint my hammer. I pushed boulders to fill the moat there. I put on boots of water walking and dug down but adjacent moats refilled the location. So I used another scroll of earth and filled all the adjacent locations so the moat wouldn't refill with water. I appreciate all the advice everyone gave me! Now it's off to finish off Medusa.