Reddit mentions of DK Eyewitness Books: Ocean

Maybe something like Eyewitness Ocean would work. I loved the Eyewitness videos and books as a kid and they’ve held up pretty well!. I know you didn’t ask for videos, but here’s a link to the Eyewitness episode on oceans to give you an idea of the content the books and videos may have if you aren’t familiar with them. I watch the videos sometimes as an adult and still find I can learn things from them!

Edit: forgot to mention, the book covers everything from plate tectonics to plankton. I remember it being pretty comprehensive as a kid

Growing up, I was always a huge fan of the eyewitness series of books. I think they're about the right age group too. They have a fish and oceans one.