Scroll of nature, which creates grass, trees, and shallow water in a radius around the player (larger if the scroll is blessed). If cursed, it summons blights. Druids should start with one or two of these so they will still be able to employ grass tactically even if they don’t start with the spell of create grass.

Water elementals’ HP (and Pw, if applicable) regeneration is boosted when they are on a water-containing space such as a moat or a fountain. Possibly, they can also suck up the water to regain a large quantity of hit points at once, which removes the water terrain if it is expendable (a fountain or pool or puddle of shallow water will dry up).

In order to make them more interesting in combat, their AI may favor hanging around sources of water and trying to find a path to the nearest water when they are injured.

When a puddle of shallow water is trying to dry up, first check whether it’s part of a larger water body. Do a breadth-first search to find all connected water terrain and see if it either reaches the edge of the map, or if it connects up to a certain threshold amount of deep water squares. If either of these conditions is satisfied, the shallow water never dries up.

In variants with shallow water, pet cats will avoid it similar to how they avoid squares containing cursed items.

Shallow water shouldn’t be a free and easy method of diluting potions infinitely. Instead, it should have a chance of contaminating or polluting the potion (or turning it to a potion of sickness), where polluted potions can’t just be re-dipped to dilute them to water or cancelled back to water. Or instead of dealing with polluted potions, just have a chance of losing the potion entirely.

Water nymphs may create a pool on their spot when killed. Items they stole from the player fall into it, of course. If in a variant that implements shallow water or puddles, those are more reasonable to create than a deep pool.

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

Assuming that shallow water is implemented: all giants should use the shallow water behavior of only wetting their feet even when walking through deep water. Also, fire giants can wade through lava, which acts mostly the same.

Add shallow water to the Gnomish Mines (not GruntHack’s deep water).