It is not fair to liken this to being held underwater. When forced to hold your breathyour lungs fill with CO2 which will cause pain, an urge to breathe and a primal urge to panic because your body has evolved the ability to sense this excess of CO2 to force you to breathe. But when breathing pure nitrogen your body doesn’t have an evolved way to detect it besides minor symptoms that you may or may not notice until you pass out.
Yes, the very idea that you will die can be emotionally distressing but this will be common to all methods of execution.