"In that when nothing gets in, when nothing happens, is it not remarkable that sleep, and various states of lower levels of consciousness are associated with seeing and hearing nothing? (As you simply could not be disturbed enough)"
The remarkable thing is not that these states are associated with seeing and hearing nothing but rather the exact opposite. Consciousness seems to be a continuing stream which only disconnects when our memory fails us.
The site I provided below gives a brief history of the Samadhi Tank or Flotation Tank (whichever you want to call it.) It's function is to eliminate sensory stimuli as much as possible. The results are interesting but not really surprising if you're familiar with lucid dreaming.
http://www.samadhitank.com/history.html