Nymphs are spirits or minor deities attached to natural objects on land, in water or even the air.
Aquatic Nymphs, sometimes known as Naiads or Water Sprites, are a sentient freshwater species found in lakes and other bodies of water. While amphibians, who presumably may have gills or the equivalent somewhere about their bodies, they take the form of startlingly beautiful women who, as Geoffrey Indigo discovered, can get a bit stroppy when annoyed. One is known to inhabit the waters of Lake Overshot.
Woodland Nymphs known as Dryads occupy trees and, on Discworld, may also be male. Rincewind encounters one named Druellae while evading Bel-Shamharoth. Druellae is not a benevolent spirit, either. Nymphomania may well have a different meaning in Morporkian.
Illustrations may be found with Google:Images, et al, but these tend not to be suitable for publication in a family-friendly wiki.