Out on the Chesapeake's Prospect Bay, blinding morning sunlight dances off the thick layer of ice that covers the freezing waters for as far as the eye can see. At the docks, local watermen are confined to dry land, their boats frozen in the ice. Then comes the Sandusky to the rescue. Throughout the eastern Chesapeake this week, the 80-foot, 120-ton, steel-hulled boat, was breaking ice and setting watermen free to do their business.