I dont think there is currently a way to measure pain objectively. It would need to deal with measuring signals sent to the brain by nerve cells. But I dont think its entirely understood how this works. There is conventionally different types of pain though.

https://www.mydr.com.au/pain/pain-and-how-you-sense-it/#:~:text=When%20we%20feel%20pain%2C%20such,and%20the%20pain%20is%20perceived.

100*1.29 [g/L]=129 [g/L]=0.129[kg/L]. Air would still be less dense than water there. The required pressure to make air the same density of water is 827 atm. However, air will probably dissolve into the water before reaching that pressure.