I read a story in the 1970s, probably older, the only part I remember was the person had his gag reflex cut so he could live in the undersea city. It was separated from the rest of the ocean only by a membrane to keep the waters separate. It had oxygenated water that you breathed. It mentioned he could not go back to land because he might choke since no more gag reflex. He might have been captive.
Could it be "Ocean on Top" (1973) by Hal Clement? The membrane separating the ocean from the breathable fluid matches.
Edit: I checked the text, and I believe we have a match. The main character is trying to infiltrate the underwater society. He is captured and agrees to the adaptation process after first asking:
'Bert, could you go back up above now if you changed your mind about staying? Or is what they did to let you breathe water impossible to reverse?'
He smiled and used the stylus again.
'We're not breathing water; that analysis misses on two counts. They did make an irreversible change, but it's not a very serious one. I could still live at the surface, though the shift back to air breathing would be somewhat lengthy and complicated.'
And then afterwards:
I tried forcing myself to breathe. I found I could squeeze liquid slowly out of my lungs and get it back equally slowly, but it hurt and made me feel even dizzier than being right side up and upside down simultaneously. The liquid went into my windpipe; I could feel it, but there was no tendency to cough. I still think that must have been one of the trickiest parts of the conversion procedure, considering the nerve and muscle activity which coughing involves.