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.