A very popular dividing line theory in North Jersey (especially the immediate suburbs of New York) is that the Driscoll Bridge on the Garden State Parkway is the dividing line between North and South Jersey.
Named after former NJ governor, Alfred E. Driscoll, most Northern NJ/ New York vacationers to the Shore consider themselves, once they cross the bridge, which spans the Raritan River, to be "Down the Shore" and can "smell the ocean." Actually, as one person pointed out, they are still in Middlesex County, smelling the Raritan Bay. But it is brackish if not salty anyway, so I think their noses are justified.
Below is my line taking the end of the Raritan River and following it along its most western trajectory . . . the black is just me connecting where the river line would be if it formed at a similar parallel starting at the Delaware River. The map to the right is the Raritan River and its branches.