The Water Elemental  belongs to a group of ancient spirits, like powerful genies, and represented the four elements: Earth, Air, Fire, and Water.  They were blamed for disasters such as fires, earthquakes, and hurricanes. 
In New York, one was originally a tiny blue pebble kept in a jar at a Magic Supply Shop above TV World. During a bust, Ray Stantz shot the ceiling and knocked the pebble loose. It fell inside a new television gifted to the Ghostbusters. Eventually, the pebble went into the circuits of the set, which caused Ray to see visions of the future. The events included the Ghostbusters being flooded on one case and a case involving the capture of Pirate Ghosts. All the while, the pebble was in the process of fusion and growing like a supernatural fungus in the TV. 
The Ghostbusters knew that things were going bad when they saw a vision of their deaths. This was prevented when the Water Elemental was born. Egon elected to try and destroy it with lightning since fire was the mortal enemy of water.  Having the advantage of knowing what would happen, the Ghostbusters prevented their destruction and destroyed the ghost. It believed it would be safe near the Firehouse's lightning rod atop the roof. Peter Venkman shot the rod with a Proton Stream, causing the Elemental to vaporize.
According to an EEG readout, the growing Elemental showed high brain wave activity. However, its overconfidence was also its downfall.
The Water Elemental has power over the water elemental and can use rain storms. It can has precognitive powers in relation to events that involve water.
- ↑ Egon Spengler (2009). The Real Ghostbusters - "Future Tense" (1989) (DVD ts. 20:03-20:06). Time Life Entertainment. Egon says: "Gentlemen, I think we're dealing with a Water Elemental."
- ↑ Ray Stantz (2009). The Real Ghostbusters - "Future Tense" (1989) (DVD ts. 20:11-20:19). Time Life Entertainment. Ray says: "Elementals were ancient spirits. Like powerful genies. They represented the four elements: earth, air, fire, and water."
- ↑ Winston Zeddemore (2009). The Real Ghostbusters - "Future Tense" (1989) (DVD ts. 20:20-20:27). Time Life Entertainment. Winston says: "Yeah, people used to think Elementals caused things like fires, earthquakes, hurricanes -- that's all superstition."
- ↑ Egon Spengler (2009). The Real Ghostbusters - "Future Tense" (1989) (DVD ts. 17:40-17:44). Time Life Entertainment. Egon says: "Fusion. It's growing like a supernatural fungus."
- ↑ Ray Stantz (2009). The Real Ghostbusters - "Future Tense" (1989) (DVD ts. 22:16-22:21). Time Life Entertainment. Ray says: "According to legend, water and fire are mortal enemies."