Setup: we have a fifth level full caster, a very large storage tank full to the brim with lamp oil, and a bunch of folks outside the tank who want the caster dead (perhaps they have swords and pitchforks).
Could our intrepid full caster scramble up the ladder on the tank's outside, then cast Water Breathing on himself and jump down the manhole in the roof to hide safely under the surface of the oil? Or would said caster drown in the oil despite the Water Breathing spell?
Best Answer
Considering that the spell specifically states that it makes the creature capable of breathing water, and no other substances are mentioned in the spell description, I'd say that the wizard would drown if he tried breathing lamp oil after casting the spell. If it was meant to make the user capable of surviving without air in any medium, I'd think the spell text would state so, without going to the trouble of mentioning one medium in particular. Since the spell description goes to the trouble of mentioning water in particular and mention makes no mention of any other uses of the spell, it seems pretty clear that both RAW and RAI points to the spell only being effective in water.