Vodyanoi

I'm not a specialist, but there are some books considered as a major source about Russian folklore, like this one (Chapter IX specifically). You may try asking Google to translate.
From what I gather, basically it's a water dwelling mystical creature, haunting a body of water. However, its origin may vary even around Russia. Usually they prefer lakes known to have traitorous nature like dangerous shores, deep torrents and such.
Sometimes it's a fallen angel landed into a lake, or, to put it simply, a devil. Usually it haunts a deep lake and does typical devil stuff. It's vulnerable to exorcism, and in some variations local villagers do not hunt catfish, since water devils use them as mounts. Somehow devils can procreate and have families. Fairytale hero Balda gets to "negotiate" with some of them.
Sometimes it's an immortal water spirit whose appearance may vary from humanoid fish like creature to a human with body covered in water dwelling foliage. In some beliefs their visible age corresponds to the current phase of the Moon (eg. he's young at young Moon and old at full Moon). They may live in lakes and rivers, sometimes they walk out to the land to do mischief, steal cattle or even enact petty vengeance on local populace (eg. for catching fish it favors or likes to eat). Physical damage on a body of a drowned often considered as result of a beating taken from Vodyanoi. Long story short, whatever bad things happen near water, including swamps, it's their doing. This led to creation of many safety rules like never get into the water at night and such.
They too can have kin. Drowned maidens are believed to become their wives. They may also act as mischievous female spirits or just plain undead.
One loose interpretation is the lonely Vodyanoi from the "Flying ship" cartoon.
Some versions just dwell in the water at their leisure, while some possess aristocratic features acting as a leader of local watery kingdom with his own palace and court.
For some reason Vodyanoi hates Domovoi. Whenever they meet, it's a fight.
Also millers who work on water mills used to have various rituals and customs to prevent Vodyanoi's from disrupting the mill.
In some cases Vodyanoi was considered as protector of bees, so beekeepers made sacrifices by dropping honey into the water and placing their hives near a lake. Later this role was given to certain Christian saints.
In some cases water spirits may be straight up undead, damned souls bound to a body of water for their sins. Reasons may vary, from political to Biblical. From undead Egyptian pharaohs in Black Sea to the murderers of Prince Andrei Bogolyubsky haunting a lake. Or even a whole city swallowed by the nearby lake as God's punishment for refusing to give shelter to travelers. Not to mention countless local legends tied to particular drown victims.
The undead do typical haunting things, like splashing, wailing, trying to kill whoever dares to sail their waters, usually, again, a lake.