Disclaimer: I understand this is a more realistic/thorough interpretation of the rules than was ever intended, yet here I am. Anyone who has no interest in discussing logic for the sake of logic, this is your stop - please exit the thread.
The wording for the spell Water Breathing is: "The transmuted creatures can breathe water freely". Does this mean that they can breath in water as a fish (breathing the dissolved oxygen in the water, exhaling CO2, just as humans do), or do they literally inhale and exhale H2O, effectively wasting nothing?
Random scenario: A human finds himself in a 10 gallon tank of water, with no way of surfacing for air. He casts Water Breathing on himself. Can he breath indefinitely, so long as he can keep the spell active? Or does he eventually use up all the oxygen present in the water, converting it slowly into C02? I am aware that an Air Bubble spell renders this scenario stupid, so let's humor me.
On average, a human breaths in 7 liters of oxygen every minute (the equivalent of 1.849 gallons). The percent oxygen by mass in pure water is about 88.9%. So in 10 gallons of water, there is about 8.89 gallons of oxygen present, and a human would be able to breath for about 4.808 minutes before he depleted it all.
The above calculations/statistics were Googled and are subject to being wrong, but it all makes sense to me. I apologize if anything is inaccurate. Is anyone still listening?
Your facts came from the internet, how could they be wrong?
I would say your latter interpretation is correct, it's allowing them to breathe in the tiny bits of air trapped in the water itself and not suffer oxygen deprivation as a result. If it were the former, you couldn't breathe OUT of the water while subject to Water Breathing. This would be stupid, so we shouldn't pretend it's an option.
As for your numbers, why would it use up the oxygen in the water any faster than it would the water in the air? Don't worry about it, plants got your back, bro.
Thank you for your input!
For the record: Water Breathing then goes on to say that you can still breath out of water while under its effects, so even with the first interpretation, you would be good.
Also, you're not using up oxygen faster, I just failed to mention that there are no plants in the tank, and therefor there is a finite amount of oxygen present.