Micah Sweet
Level Up & OSR Enthusiast
Nah. I'll just stick with Level Up's heritage traits and choice of heritage gifts to handle that. Far less twisting the rules that way. Neither 5e nor 5.5e are designed to do what you want, at least so far.In 5e mechanics, waterbreathing makes more sense.
A slot 1 spell could work, but the cantrip is better for an always on effect.
The old school slot 3 spell seems crazy now.
In any case, worst comes to worst, explicitly describe, "a cantrip can be Darkvision or Waterbreathing instead".