There are hardly any undines in Hitherby Dragons. That’s because they come from a different metaphysical tradition!
If you’re reading about an undine, you’re reading about water that differentiates itself from earth, fire, and air, but is one (in a fashion) with all water—water pulling, conceptually, away from everything else in the world. But in Hitherby Dragons, most water spirits are mer-nymphs, participants in the world: creatures of the rivers and seas, who turn the hard edges of things into an endless wash. They are water that permeates, water that exists with, water that dissolves at its edge into other things.
Undines are spirits of elemental water.