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.
Sender here. It may not have been a YA book, but had teen/kid protagonists. If it wasn’t YA, it didn’t have anything clearly adult-oriented in it and could lead a librarian to mistakenly shelve it in the teen section.
It’s not “Ingo”, “My Sister Sif”, or “Ultramarine”/“Rainbow and Mr Zed”.
I think it’s the neptune project!!! I’ve been wracking my head trying to find the name and searched so many keywords for something of this description- yes, the netune project by polly holyoke!!
Could it possibly be “Crisis on Conshelf Ten” by Monica Hughes? This definitely is a YA, published 1975 but was still in print in the 1990s. There are characters with gills (although the protagonist needs a diving suit), siblings, and a visit to a deep chasm followed by a dramatic surfacing. At least two of the covers the book had over the years are similar to what you describe, especially the original one, but I don’t know if the art wrapped around.
It sounds like Dr. Franklin’s Island by Gwyneth Jones.
Has a ocean cover, the main character is turned into a sort of half fish half person creature via genetic engineering. Definitely a creepy book.
Hi! I found your post while searching for the same book. Found it thanks to Reddit! It’s Splashman by Lydia Weaver (or at least the one I was looking for is this.) there is a picture of the cover on Goodreads and it’s exactly as you describe (and as I remember).