For a moment, you saw the shadow of an otherworldly being on the bottom of the lake.
There are rumors of a human man who fell in love with Scylla, a beautiful mermaid. Because they lived in different worlds, the man asked a witch to cast a spell so that he could be with his love.
However, the witch took a liking to this man, and instead cast a spell on both of them to destroy her mermaid rival. The beautiful mermaid was transmogrified into a monster, the man lost his memory, and the witch wooed Scylla's lover.
The mermaid still waits at the bottom of the lake for the day she encounters the two again.