Looking for picture book loosely based on The Little Mermaid. In this version, the mermaid has two tails instead of one. She meets and falls in love with a sea captain, and when he leaves she gives him a lock of her hair to wear around his finger. She cries on the rocks like her grandmother did once after he leaves. The captain forgets about her but encounters a witch later on who reminds him of what he promised, and I think it ends with him jumping overboard.