Land-dwelling druids can figure out a way to use magic in order to breathe underwater (, provided that they are able to cast 3rd-level spells). But what about druids that are already able to breathe underwater, but not on land? Of course, they would have no need for the water breathing spell. (Unless they have some friends who rely on air). Instead, they would probably be more interested in a spell, which allows oneself to breathe air.
I am considering homebrewing a spell that would be 100% identical to water breathing, except instead of saying 'the ability to breathe underwater' it would say 'the ability to breathe air'.
Would a spell like that be more, less or equally powerful as water breathing. Consequentially, would such a spell be 3rd-level as well, or should it require a lower/higher level spell slot.
My players will encounter such a druid and I have to estimate how mighty this NPC has to be, in order to cast such a spell.
It should probably be a 3rd level spell
There aren't really any good guidelines for this. Spell effects are normally the reference frame for other effects and custom spells and it is not very clear what kind of "empowerment" this is.
The best analysis we can do is for what the spell effectively does, which is allowing access to a area across the water-air barrier (without the risk of suffocation). Thus the "breathe air" spell would have exactly the same strength as the water breathing spell and should be of the same level.
From this point of view, a higher level spell with a comparable effect would be plane shift (7th level) which allows access to other planes which is quite clearly a more powerful effect.
A piece of advice from the DMG you proposed spell arguably runs afoul of is:
However as this spell is intended for an NPC specifically it should not be much concern.
As a side note a spell which only allows the caster to breathe air should - for the reasons given above - be a 2nd level as it is then more comparable to alter self (which as one of its modes can give the caster water breathing and a swimming speed).