title_body: Which IPA phones can be made and understood underwater?
upvoted_answer: From reading the answers to the Worldbuilding SE you reference, I would draw the following conclusions:
  
  
  anything unvoiced goes out of the window. So no /f/, /p/, /k/, /t/, /s/ etc. They are pretty useless, as they are predominantly in the higher frequency ranges (especially the fricatives) or very short and without much energy (which would be provided by the glottis). And higher frequency sounds travel less well in water.
  consonants in general are either short (plosives) or prefer higher frequency bands (fricatives), so apart from /r/ and /l/ (and possibly /v/, /z/) would not carry well underwater.
  vowels seem best suited. And looking at the frequency characteristics, those with lower formants seem slightly preferable, so /u/ and /o/, and the back /a/: vowels produced at the back of the vocal tract. The fronted vowels (/i/, /e/, frontal /a/) would have a higher frequency component again.
  
  
  So in summary I would think a vowel-based phoneme inventory might be best. This inventory could be supplemented by some approximants (/l/, /r/, /j/, and /w/). Which is weird, as consonants usually carry most information in natural language! 
  
  Compare:
  
  
  Whch s wrd, s cnsnnts sll crr mst nfrmtn n ntrl lngg!
  i i ei, a ooa uuay ay o ioaio i aua auae!