Breathing water vapour
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.