Humans can't really "breathe" oxygenated liquid. Our diaphragms aren't strong enough to move sufficient liquid in and out of the lungs, so it can only work with external mechanical ventilation. This is occasionally used as a salvage therapy for patients hospitalized in critical care but is totally impractical and unsafe for any sort of diving.
In the real world outside of sci-fi movies, any human diving much deeper than about 0.5km will have to be done in an an atmospheric diving suit.
