For what reason does Goodwife Cruff suspect Kit might be a witch?

Goodwife Cruff believed that Kit was a witch because she didn't drown when she dove into the ocean after Prudence's doll.

Nat laughed shortly. "'Tis certain she expects you will curdle hers," he answered. "She has been insisting to my father that you are a witch. She says no respectable woman could keep afloat in the water like that."
"How dare she!" Kit flared, indignant as much at his tone as at the dread word he uttered so carelessly. "Don't you know about the water trial?" Nat's eyes deliberately taunted her, "'Tis a sure test. I've seen it myself. A true witch will always float. The innocent ones just sink like a stone."