Your body has automatic reflexes to wake you up if you start to inhale water or lack oxygen. If he didn't wake up, then there was probably some underlying health issue that actually put him into an unconscious state.
That's true for sure but isn't it also deadly to inhale water into the lungs? I think I saw it's like 6 Tbps of water can kill you when inhaled. It feels like it's possible that that happened and, since it's dark and "sensory deprivation" happened that he couldn't figure out what was going on (since he had just waken up) and ended up inhaling more? I'm not really certain.
This is not uncommon:
https://www.reddit.com/r/Drugs/comments/3yrulm/took_ketamine...
https://www.getreading.co.uk/news/local-news/floatation-tank...
https://www.ncbi.nlm.nih.gov/pubmed/20533982