What's the water goddess?

As others have said, specifics would be nice. However, I think if you are looking for a water goddess who personifies many of the qualities of water, you might look to my namesake, Kebechet.

Kebechet is a goddess of Kemet, or ancient Egypt, who personifies the embalming fluid used on the khat, or physical body, of the dead. " Her name includes the root of the ancient Egyptian word "kbch" which means "to offer libations" or "to purify" and the root of the word "wt" which refers to the place of embalming. However, her name also resembles the word "qebeshu", which means "cold water". As a result, her name is usually translated as "cooling water"."

As a daughter of Ienpw, she could easily be dangerous at times, especially considering that she often was represented with the head of a snake, an animal that was both dangerous and beneficial to existence. If you do some good, hard work researching up on her, I doubt you will be disappointed.

All that said, I think you need to look into the paths that interest you the most, research into their goddesses, and find one for yourself. Or perhaps if you wish to include all water goddesses, you should simply call her Goddess or even Water Goddess, in much the way Wiccans do with the Lord and Lady. You can always take this route until you find just the right goddess for you.

http://ancientegyptonline.co.uk/kebechet.html

There is a Yu'pik sea goddess named Sedna.

as said, there is more than one water god or goddess, it depends on which religion you are asking about. there is also no one all powerful water goddess.

regarding spirits associated with the element, Nymphs are commonly associated with water.