I agree that allowing "stretching the rules" for the sake of awesome can make the game more fun. On the other hand I want to point out that wizards are really damn powerful already, and letting them stretch the rules in this way will make that imbalance worse. Casters are going to have many more opportunities to benefit from this sort of thing than non-casters.

What does "RAI" used above stand for? (Do you really mean to suggest that the spell was intended to work in non-water?)

@DanB -- casters don't need to do creative things with their casting to IWIN at 3.x -- it's just a lot more fun for the table when the casters are being cool and creative, instead of repetitive and tiresome. Also -- RAI really is the question here -- the RAW could very well be that way simply because nobody at WotC ever imagined a cleric wandering around the bottom of a loaded oil tanker, and not because WotC wanted to actually prohibit characters from being able to breathe while submerged in other liquids. (besides, RL SCBA gear, AIUI, will let you breathe with your head stuck in oil)

@SevenSidedDie -- likewise, we have no idea that Wizards intentionally wanted it to stop functioning in not-water.

@Shalvenay Except for literally saying so?

@Shalvenay The DMG mentions drowning as a consequence of being engulfed in quicksand, duststorms, acid, and lava in addition to plain, old water.

@HeyICanChan -- is it reasonable to leave a character drowning in something they have no countermeasures against? floods room of dungeon with vinegar (Besides, IRL, you can drown in corn.)

@Shalvenay It depends on the campaign. Grimtooth's Traps (1981), for example, memorably puts forth killing adventurers by deliberately filling pit traps with unusual substances (e.g., hot cheese, whipped cream). If the DM's created a world where trap-builders know wizards can breathe water and a trap-builder can but doesn't use vinegar, is the trap-builder an idiot, the DM going easy, or what? That becomes a play-style question, not a rules question.

I would have thought is was obvious that you allow the spell to work in liquids that (a) can be expected to hold dissolved oxygen, (b) have a viscosity not too much above that of water and (c) won't kill another way. So vinegar is in and beer, but you will get alcohol poisoning from hard spirits. Lava is out as there is unlikely to be dissolved oxygen even if the character is fireproof. Molasses is out because you simply can't move enough of it around. I don't know about lamp oil and lacking a chemist in the room would probably allow it, but seriously you can over do the rule of cool.

@HeyICanChan it's much easier to get 1000 gallons/five foot cube of water than of even relatively tame liquids like vinegar. With a wild guess at price of 5sp/gallon, selecting water instead of vinegar is a significant cost swing, and "defeated by a single wizard spell" is not the strongest test for uselessness. Also, vinegar is best described as "tainted water," (equivalent to salt water, but with different ions), and so is not a good test case.

@fectin So by RAW, you can suffocate a wizard who's using this spell by tainting the water? What percentage of vinegar in water makes it stop being "impure water" and start being "weak vinegar"? :)

@Adeptus As written, it says that you can "breathe water". From various sources, I think the intent includes salt water. Google tells me that salt concentration in salt water is 35 parts per thousand. That suggests that any water solution less concentrated than that and not otherwise harmful is fair game. For water based acids, that suggests any pH > 1.45ish is fair game for water breathing. However, just like mustard gas, the issue quickly becomes burns, not breathing. Lemons and strong vinegar are around pH 2.5, and that's about where I'd start looking at low damage/round.

@fectin I was building off your comment to push against the "RAW says water only" argument. Hence my question, how much non-water-liquid (vinegar, milk, fruit juice, whatever) do you need to add to water before it becomes non-water? :)

@Adeptus I don't know. But if the spell is intended to let you breathe under the ocean, the minimum is more than 3.5% other stuff.

@fectin -- I also wonder if it's intended to let you breathe under the surface of hypersaline lakes/ponds -- Earth's current record holder sits at a whopping 44% salinity.

@SevenSidedDie - When writing the spell, the authors intended it to allow characters to go on underwater adventures in flooded ruins, or under the sea. Not to hide in water tanks from enraged populace. From many descriptions of the authors of the PHB, however, especially the test games they ran, their intention was not to be literalist and harshly proscriptive about interesting uses of abilities. They'd say it was purely up to the GM, with the intimation that the GM should say 'yes' if it was interesting and story-appropriate, and 'no' if the player was simply trying for advantage.

@fectin - Energy Immunity (Acid) + Water Breathing = breathe water-based acid? Be a high level wizard, fill your home with acid, cast Extended/Persistent versions of those two spells on yourself every day. Alternatively, cast it on some sharks, fill a pit of acid with them, drop adventurers into it.

@JackLesnie that all seems very reasonable to me.

Would you be equally quick to walk away from this campaign were the DM first to make clear that—or even to demonstrate (maybe using an enemy wizard) that—, despite being affected by a water breathing spell, one can drown in a tank of not-water?

@HeyICanChan Yes. Because the GM has intentionally trapped the wizard player in a scenario where they've 'already shown' the wizard can't use that spell to get out of it. Which implies an extremely high degree of railroading, and is a classic sign of 'gotcha!' GMing. The possibility of that occurring accidentally is so minuscule as to be completely ignored.

@JackLesnie Ruling that a spell doesn't do certain things outside of what its wording specifically says isn't necessarily railroading. I've known GMs who'd say "water breathing won't let you breathe the acid, unfortunately, but you remember seeing a scroll of acid breathing for sale back in town." And then, while the players discussed whether swimming in the acid was worth their time, the GM would come up with an interesting thing to put at the bottom of the acid pit so that there was something for the players to find once they reached their decision.

I'd be pleased to see a citation of these "many descriptions" that the spell was intended by the designers to work for non-water. Equally I'd be pleased to see cited these "many descriptions" that the designers intended the rules to generally be ignored when desired. (Also, you may wish to remember that there is no evil DM railroading involved—Shalvenay's question setup is hypothetical.)

I am not a RAW sort of player, but water breathing has always been water breathing and in our games it was restricted to water that other things could live in. I like the inventiveness of the wizard, but no, breathing lamp oil is not water breathing.