Have some water demons, moccasins and nymphs appear on the Plane of Water, in order to imply that this is the dimension from which they’re being summoned into fountains.

You can’t dip successfully for Excalibur if water nymphs are genocided (and maybe extinct, as well). Not because you killed the Lady of the Lake, but because she’s mad at you.

For variants with a “frozen in ice” status effect, this effect will be randomly inflicted a small fraction of the time on water-ish monsters (water elemental, water nymph, water troll) that get hit with cold damage.

If a wood nymph is near a tree, or a water nymph is near a fountain, they won’t wander very far from it. Specifically, their AI would tell them to consider any space more than X distance from a fountain to be invalid.

They also don’t teleport away from fountains, unless they’re scared or carrying stolen items. When they do teleport, their destination is biased towards any trees or fountains that are on the level, respectively.

“You hear the splashing of a naiad” may happen if there is a water nymph on the level at all.

Water nymphs may create a pool on their spot when killed. Items they stole from the player fall into it, of course.

Wood nymphs may create a tree on their spot when killed, and mountain nymphs may leave a boulder.