The Water God is a character that debuted in Mortal Kombat Mythologies: Sub-Zero and acted as one of the bosses.
He was one of four guardian gods assigned by Raiden to guard the Amulet of Shinnok in the Temple of Elements and acted as the god of water in the Mortal Kombat universe. He possessed the ability to fire jets of water and teleport. He was defeated by the elder Sub-Zero, allowing the ninja access to the fire area of the temple.
In the official Mortal Kombat 4 comic that came with the PC version of said game, the younger Sub-Zero attempts to make peace with the temple guardians, but discovers that they are gone. This could indicate that the Water God was killed by Shinnok during his invasion, an idea further substantiated by Fujin's Mortal Kombat 4 biography screen stating that he and Raiden are among the few surviving Earthrealm gods.
The Water God appears in the film Mortal Kombat: Annihilation (as female, hence Water Goddess and Elder Goddess), discussing with the other gods (Wind God and Fire God) about Earth's fate. During the climax of the film, she and the Fire God arrived to stop Shinnok's treachery and declared Mortal Kombat. After Liu Kang and his friends defeated Shao Kahn and his warriors, the Elder Gods imprisoned Shinnok and have his son Raiden take his place.