The Lucky Lobsters — Gary L. Dorion

When Leo and his family celebrate his birthday beneath their pink coral reef, he has no idea of the adventures awaiting him later that day after Lena and Libby, his childhood friends, persuade him to go exploring in a deep green kelp forest. There they become lost, take the bait in a lobster trap and panic turns to terror. The trap rises and they enter a world they have never seen. Fortunately two kind humans buy them from a grocery store and set them free.