Water Ghosts are a collective of over 40 ghosts that try to sink Manhattan.
During a summer season, ghosts in New York haven't been seen for two weeks. A large group of ghosts found the legendary Pillar of New York, an ancient construct that holds up Manhattan Island, and blocked its flow of ectoplasmic lubricant. Without the lubricant, the Pillar would crumble and cause chaos for the world. A series of earthquakes are caused by the build-up of ectoplasmic lubricant.
The dangerous pockets of energy created by it had the potential to influence dramatic warming trends across the planet. The Ghostbusters investigated and noted the unusually high concentration of ectoplasmic residue. Since they weren't hired, the Ghostbusters didn't progress further. However, Ray was eager to learn more and ventured underground with Slimer. Hopelessly outnumbered, Ray sent Slimer to summon the others. The Ghostbusters chased away the ghosts and restored the Pillar to normal.
The Water Ghosts are not to be confused with Undines.