I've already established that neurogel acts as a ventilation medium, and the widespread use of recreational suspension implies that the general public is comfortable with the concept of liquid breathing.

So instead of scuba gear, yinrih divers fill their lungs with a fluid similar to but distinct from neurogel, which allows them to dive extremely deep and ascend back up without worrying about decompression sickness.

Some possible follow-ups are that they have to have a CO2 scrubber attached to a major artery like a dialysis shunt, that they have to be intubated with a similar device that absorbs CO2 directly from the lungs, or that the liquid causes respiratory issues if one is exposed to it over a long period of time like asbestos. Whatever it is, the surface dwellers of Sweetwater will be immediately recognizable for possessing this feature (a scar from where the CO2 scrubber was implanted, or a chronic cough from exposure).