I'm just curious, frankthedm: is it your belief that the game's designers actually intended that water breathing cause death, nausea, or some other harm upon being dispelled and/or its duration expiring, or do you just personally think that's a good idea?Even the spell's Name implies that.
"The transmuted creatures can breathe water freely."Why is water going into lungs in the first place again?
Exactly. And you don't fall like a feather bobbing sideways and subject to small gusts of wind when you cast featherfall.To interpret a spell named Water Breathing as literally as to mean it allows you to breath water makes about as much sense as saying a spell called Stoneskin turns your skin to stone. And thus subjects it to Stone Shape or Transmute Stone to Mud.
The very nature of breathing makes it logical a spell that lets you breath water would put water into your lungs.But there's no logical reason to assume just because you're "breathing water" that the water is actually going to a pair of organs that can't handle water.
I have to admit the growing gills idea for the spell is kinda cool and actually sounds like it would take less "magical energy" to put gills on an airbreather than total magical conversion of the lungs.![]()

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.