water-witch

A grebe

... bird was never seen again, dead or alive. There was a mystery about it , and when some one older and wiser than he told him it was a water witch or hell diver, the mystery was only half solved. Where did it go? How could it stay[…] Fortunately for the dabchick, […]

[…] I knew that another pair of Grebes had chosen this secluded pond for their home. Careful search revealed only a deserted or incompleted nest, and I continued until I came to a weedy stretch. Examining it with binoculars, before entering, as it was quite open, I espied another of these elusive water-witches upon its nest.

A grebe:

The Hell Diver seen was probably the Pied Bill Grebe, […] it surely is a water witch and has disappeared.

... Grebes are highly aquatic expert divers but labored fliers. […] a "water-witch" [Blanchan 1902: 12]. The Pied-billed Grebe is represented at CA-44.

The storm petrel.

Ten or fifteen dark little birds […] these are "Mother Carey's chickens," the "water-witches," Stormy Petrels, which are so familiar to the eye of the sailor, and the sight of which he dreads so much; […]

[see chapter title, viz.:] Song of the Water-Witch (The Stormy-Petrel)

Used other than figuratively or idiomatically: a witch who lives in, controls, is able to dowse or find, or is otherwise connected with, water.

[…] plays with the stormy petrel, / And answers the sea-gull's call. […] the water-witch, gray and ancient, / And hearing the tales she tells.

The water-witches of Meterik inhabit the islands. There are fifteen of them. They argue all the time over minutiae. They throw magic spells at each other for fun. Normally they show not too much interest in humans. That changed[…]