Water Mephit

(from Planescape: Monstrous Compendium Appendix I - 1994):

These thin, finny humanoids are covered with sea-green scales, and webs of skin connect the spines of their ears, toes, and wings. They have fishy, staring eyes and fish lips. An odor of brine accompanies them, and they drip salt water. 

Manifesting an irritating joviality, water mephits make remarkably tactless comments on their companions’ actions and situations: “Buck up, you can handle these fiends. Or if not, you’ll make good dretches.” They attach themselves to adventuring parties (unasked) out of an appetite for novelty. 

Combat: Water mephits can attack with two claws (ld3 damage each), but they prefer their breath weapon, a stream of acidic greenish water (2d4 acid damage for two rounds; one save vs. breath weapon halves damage on both rounds). The mephit can breathe every other round, automatically hitting one target. 

The semi-liquid forms of water mephits are immune to cutting or impaling damage from nonmagical weapons and to fire damage (including magical fire), but they take double damage from cold attacks. Of course they can breathe water, and they can drink any liquid, such as mercury or poison without damage. They regenerate 1 hp per round spent only drinking liquid. 

Water mephits can attempt to gate in one other water or ice mephit once per hour. 

Ecology: Captured water mephits stand watch in kitchens, ready to put out fires, or maintain aquaria for the wealthy.

(From 3.5e Monster Manual I - 2003):

This winged creature looks like a miniature fish-person. It is covered with scales and views the world through black, bulbous eyes. 

Water mephits come from the Elemental Plane of Water. Water mephits are jaunty creatures with an unflagging sense of humor who quickly get on the nerves of everyone around them. Each one is about 4 feet tall and weighs about 30 pounds. 

Water mephits speak Common and Aquan. 

Combat:

Breath Weapon (Su): 15-foot cone of caustic liquid, damage 1d8 acid, Reflex DC 13 half. The save DC is Constitution-based and includes a +1 racial bonus. 

Spell-Like Abilities: Once per hour a water mephit can hurl an acidic blob that functions like Melf ’s acid arrow (caster level 3rd). Once per day it can create a mass of smelly fog that duplicates the effect of a stinking cloud spell (DC 15, caster level 6th). The save DC is Charisma-based. 

Fast Healing (Ex): A water mephit heals only if it is exposed to rain or submerged up to its waist in water. 

Skills: A water mephit has a +8 racial bonus on any Swim check to perform some special action or avoid a hazard. It can always choose to take 10 on a Swim check, even if distracted or endangered. It can use the run action while swimming, provided it swims in a straight line.