John Bauer, Agneta and The Sea King, Swedish Folktales.
This has a beautiful story that goes with it, with the sea king trying to convince her to just take his hand and she’ll become the water queen and have mermaids in waiting and pearls in her hair
and she does not reach out
because she knows that he is the king of an underworld and he’ll take her down into the cold dark depths
and she won’t return