The Collaborative International Dictionary of English v.0.48:

Innavigable \In*nav"i*ga*ble\, a. [L. innavigabilis : cf. F.
   innavigable. See In- not, and Navigable.]
   Incapable of being navigated; impassable by ships or vessels.
   --Drygen. -- In*nav"i*ga*bly, adv.
   [1913 Webster]