Motion sickness or, more correctly, motion maladaptation syndrome is a condition that occurs when people (as well as fish and other animals) are exposed to real or apparent motion stimuli to which they are unfamiliar and hence unadapted (Benson, 1988). It has been recognised for more than a century that the term 'Motion Sickness' is a misnomer. Motion maladaptation syndrome is merely an indication of intact vestibular function and, as such, should be regarded as the norm rather than the exception...
The classic theory explaining the causation of motion sickness symptoms is that of Neural Mismatch. The theory is based on work from over a century ago (e.g., Irwin, 1881; James, 1882; Pollack, 1893) but it has only gained wide acceptance since it has been promoted in publications by Reason (1970, 1978), Reason and Brand (1975) and Oman (1982). Neural mismatch generally occurs when there is a conflict between signals received from either the visual system, or the vestibular systems and other gravireceptors. However, it can also happen when these signals differ from those expected by the central nervous system (CNS). It can also occur due to conflicts within the vestibular apparatus, i.e., between the semicircular canals and the otoliths.
I donno. Fish don't worry all that much about falling. It's only us land-dwellers that need to be concerned. My immediate response is that if any such creatures were oblivious to falling, they might not survive all that well. We're the descendents of those who got it right.