Breathing water vapour Normal air consist of oxygen, CO₂ and nitrogen, with traces of water etc. Now imagine displacing all gasses except oxygen with water vapour. By water vapour I don't mean hot steam or fog. Just gaseous water. The question is would it be beneficial or harmful to human, and how much so? And assuming it's not deadly, in which circumstances could it be useful? One thing I am wondering in particular is deep dive and high altitude applications. Having no/low content of nitrogen in such an artificial air mixture would help to avoid decompression sickness. And one would think water vapour is at least cheaper than pure oxygen (also might be safer and have less side effects).

Just because you have removed the Nitrogen and CO₂ won't help you keep a higher vapour pressure of H₂O in the air. You Could REMOVE all the other gasses and just have O₂ at its normal partial pressure at 1/5 bar total pressure and breath fine, but there would be a fire risk in this environment due to no non O₂ molecules to quench excited O₂.

The only way to have a high enough vapour pressure of H₂O for something like diving would be to heat it, but I don't think a lungful of very hot water vapour would be terribly good for you.

You can't just pressurise water vapour to any pressure you feel like; it will condense fairly easily.

To avoid Nitrogen's toxic effects, divers often use Helium to replace it as pure Oxygen is deadly at high pressures.