The Pout-Pout Fish and the Big-Big Dark by Deborah Diesen

Mr. Fish wants to help his friend Ms. Clam when she loses her pearl, but though he's fast as a sailfish, as smart as dolphin, and as strong as a shark, Mr. Fish has a secret: he's scared of the dark!
Very young children will swim along with Mr. Fish as he journeys deep into the ocean to new and mysterious places. They will discover, as Mr. Fish does, the power of friendship to light the way through the big-big dark.