Water

Water dragons are potentially the easiest recognisable dragon element. They are aquatic or semi-aquatic, never straying far from water, are either incredibly large or incredibly small, rarely have the ability to breathe fire.

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Discovered Species

Anglertooth
Aquasoar
Brinebelly
Geosaur (Water/Earth)
Labyrinthia
Potbelly (Earth/Water)
Reefsnapper
River Ridgeback
Sawtooth
Spitfire
Squirtling
Terrafin
Thorpedo

History

Water dragons were established long before any other elements were. At this time, dragons were split into two: earth dragons and water dragons. Earth dragons just referred to every dragon that lives on land or in the sky, and water dragons referred to any creature that lived in water and breathed some sort of fire, water, or other material.
Because most creatures in the oceans didn't fit exactly into the idea of a "dragon", people just named them water dragons to make things simpler. It was only then when the dragon elements were established was the water element declared, alongside fire, earth, light, lightning, ice, and air.