Why can a ship not use the internal energy of sea water to operate its engine?

Answer

For using the internal energy of sea water to operate the engine of a ship, the internal energy of the sea water has to be converted into meachanical energy. Since whole of the internal energy cannot be converted into mechanical energy, a part has to be rejected to a colder body (sink). since no such body is available, the internal energy of sea water cannot be used to operate the engine of teh ship.