PHILADELPHIA -- The 990-foot SS United States served as a geographical marker of sorts on the Delaware River waterfront in South Philadelphia for almost 30 years. That is slated to change when the vessel is towed to Mobile, Alabama, for a final cleaning before it’s sunk into the Florida Panhandle. It is set to become the world’s largest artificial reef and will be turned into an underwater tourist attraction. To followers of the ship, the departure may feel anticlimactic and unceremonious.