I agree with the general sentiment that there are limits to what should be possible even with the rule of cool.
In this specific case we don’t even need to go into the territory of undefined stuff that the DM decides on the fly, the rules as written already explicitly say “You create up to 10 gallons of clean water within range in an open container.”
“open mouth” as an open container is already a stretch.
“open mouth all the way down the lungs” as an open container is just bullshit. Rule of Cool definitely does not apply. That is not creative or cool in any way.