In oceanography the lead can be two different but related things:
By one definition it is the narrow strip of ice-free water at the edge of a
glacier or ice shelf, between the ice mass proper and the surrounding sea ice. Ice masses, due to their inclination and the temperature difference between their surface and the water, often cause persistent katabatic winds that drive the loose ice fragments nearest the edge of the ice mass further out, leaving a strip of virtually clear water. On satellite images this is what gives those features a dark edge.
It is also the name (as Webster briefly states) for a navigable passage or a fracture in the ice that can be used by surface vessels, whether it be open or frozen over again little enough to be broken.