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.

You can quaff an adjacent water elemental, giving you a YAFM (“The water elemental looks deeply offended.”) and angering it if it was peaceful. Drinking it does not have any effect on you, but deals 1 HP damage to the elemental, killing it if it was at 1 HP (“As you drink the water elemental, it disintegrates into droplets.”)

You can also dip potions into a water elemental to dilute them, which also produces the offended message and angers it, but does not do any potion effect or damage to it.

Whenever a fire attack “partially vaporizes” a water-based creature such as an elemental, there is a chance a steam vortex will be spawned adjacent to it.

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.

Water elementals have an active rust attack.

Fire/water elementals are not only uninjured by fire/rust traps, but are strengthened or healed by them.

Water elementals deal active water damage to the player, more broadly than rust monsters: blanks scrolls, dilutes potions, extinguishes candles, etc.