The Collaborative International Dictionary of English v.0.48:

Depthless \Depth"less\, a.
   1. Having no depth; shallow.
      [1913 Webster]

   2. Of measureless depth; unfathomable.
      [1913 Webster]

            In clouds of depthless night.         --Francis.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "depthless":
   ankle-deep, cursory, epidermal, few, footling, inconsequential,
   inconsiderable, insignificant, jejune, knee-deep, light, little,
   low, meager, miniature, negligible, no great shakes, not deep,
   on the surface, petty, picayune, picayunish, shallow,
   shallow-rooted, shoal, short, sketchy, skin-deep, slight, small,
   superficial, surface, thin, tiny, trifling, trivial, uncritical,
   unprofound