Okay so i have this world that is inhabited by an alien race that outwardly is basically undistinguishable from an ordinary earth-human. Due to the specifics of this planet they live on they can breath just fine in air that would be way too rarefied for normal humans (So Heights are no problem at all for them) and due to their world having even more water than our earth also caused them due to evolutionary circumstances to be able to hold their breath for very, very long.
I kinda want them to have the ability to also breath in water (this ability has by the time my story takes place been neglected for so long most have forgotten that they actually have it, i plan on having it rediscovered due to the ability naturally kicking in when one of these humans is in danger of drowning due to not being able to surface before they can no longer hold their breath at wich point subconcious survival instinct basically kicks this ability into gear)
The thing is i basically want them to be able to breath both air and water just using their mouth and/or nose and lungs. Is this possible or would they need some strange biology like and extra set of lungs for waterbreathing?
Edit: If this is as impossible as it seems to be, could i perhaps get away with them having an extremely well developed/efficient diving reflex? I just need a way to get them to be able to be underwater a long time (compared to normal humans) without needing diving gear that does not entail me giving them gills or turning them into mer-people