345P: YA book where kids get gills cut in their necks and go underwater

A group of kids, maybe a girl and her younger siblings, are surviving on their own. They meet a boy on the beach, who cuts gills into their necks somehow, and they go swim in the ocean and visit a deep and dangerous chasm at one point. Mermaids may or may not be involved. They leave the water in the end. The cover is blue with a picture of them swimming; the art wraps around the front and back, though it was probably a paperback. YA Pre-2000. Eerie feel.

323L: Mermaid short story/book of ironic stories

I believe there was a YA book of ironic stories that I read between 1979-1983. Story I’m looking for: A teenage boy discovers a mermaid living in a pond at the bottom of a quarry. She never breaks the surface of the water, but looks beautiful underneath it. The quarry will soon be dynamited and caved in, so he decides to save her and move her to another pond. When he does capture her, with much difficulty, she is a hideous monster (the water had distorted her features) that nearly kills him. He lets her stay in the pond. The next day the quarry is dynamited and she is buried under the rubble. Have asked for years on every search site I could find. Please help! Thank you!