If you can get your ship through, then both the ship and the waterway are navigable.
If people of average intelligence could wade their way through the federal tax code without help, the IRS would be navigable. Alas. On the other hand, the New York City subway system is quite navigable, as are any of its functioning trains. If one can get through a passageway or system, it is considered navigable. Likewise, if a truck, car, boat, plane, train, or other vessel can be steered then it, too, is navigable.