Lost Mine of Phandelver
Olhydra, Prince of Evil Water, appears as an endlessly breaking wave of water, an amorphous blob with almost-human features buried within its depths. She is about 20 feet wide and weighs 5,000 pounds. She may also appear as a feminine, half-human and half-pleiosaur creature with tentacles for hair and a semi-transparent liquid body.
Olhydra lives in the ruins of a castle made of living black coral, submerged in a great indigo lake, on the Elemental Plane of Water.