Start with high-confidence fishes books, then narrow by subgenre or adjacent topics.
Ocean: a Peek-through Board Book
Britta Teckentrup
Wish for a Fish: All About Sea Creatures
Bonnie Worth
Deep in the Ocean
Lucie Brunellière
Sharks (national Geographic Kids Readers, Level 2)
Anne Schreiber
Weird Sea Creatures (national Geographic Kids Readers, Level 2)
Laura Marsh, National Geographic Kids
The Ultimate Book of Sharks
Brian Skerry
1,000 Facts About Sharks
Sarah Wassner Flynn
Fly Fishing
Cindy Jenson-Elliott
Oceanarium
Loveday Trinick
Who Would Win?: Underwater Battles
Jerry Pallotta
Smithsonian Kids Sharks
Jaye Garnett
The Ultimate Shark Field Guide
Thomas Nelson, Julius Csotonyi
Uncover a Shark
David George Gordon