This was a children’s fantasy book from the 1990s for older children about a group of human explorers on a submarine or other exploring vehicle who become wrecked in underwater caverns with lush vegetation and waterfalls which are inhabited by an advanced society of intelligent dolphin people with three fingered hands instead of flippers. In one memorable scene, a dolphin doctor works with octopi on his hands to perform a delicate medical procedure. It had a bit of a “Journey to the Center of the Earth” vibe and plotline. The cover of the book had an image of people and dolphins against a lush jungle and water backdrop, maybe with a submarine on it too.

This was a wide format, 30-50 page hardcover children’s fantasy book illustrated with full-page and full-color realistic style acrylic or oil paintings. The book was in the exact same dimensions, thickness, and rectangular format as the original hardcover Dinotopia books, and purchased around the same time in the 1990s. However, it does not appear to be by Dinotopia author James Gurney – instead someone else creating a book in the same style and perhaps for the original publisher, Turner Publishing?

Edit: Oh gosh, with a bit more sleuthing I actually solved this one myself! In case anyone else ever asks, the solution was The Secret Oceans by Betty Ballantine, as noted in the description “in the tradition of Dinotopia”