Water Mimicry – The being has a body made of water.
Hydro-Man (Marvel) is a supervillain that is more invulnerable due to being made out of water.
Electricity takes out the Hydro-Man in Occupy Avengers #2.
Naiad (DC) is a water elemental (Who’s Who in the DC Universe #2).
Chammy (Marvel) can turn into a puddle of water as demonstrated in Rocket Raccoon and Groot #10.
Weird Water Tomahawk (DC) is born in Tomahawk V1 #100.
Aballin – TSR 2145 Monstrous Compendium Annual Volume 1
Killian Bra’nanche – Aspen Universe – Sourcebook #1 (2016)
The Blue – Aspen Universe – Sourcebook #1 (2016)
Water Archon – D&D 4th Edition – Monster Manual 2
Water Elemental – World of Warcraft Monster Guide
Trading Card Games have beings made out of water.
Aballins – WTC11832 – Monsters of Faerun
Is Hydroman the first comic book character to demonstrate this power in Heroic #1 (Eastern Color, 1940)?