This is an illustrated book from the 80’s or more likely the 70’s. It entails a boy who rows his dinghy into the ocean. He spots something on the sea floor and dives down to find a treasure chest. He discovers that there’s a hole in the bottom of the chest so that there’s nothing in it save for a single doubloon. That either happens in the boat or back at the harbor — I think I recall an older sailor explaining the doubloon to him. It had beautiful art — especially the water refraction effects. I would love to show this book to my son.