Underwater Counting is a number book and a concept book. I waited a bit to give this book to my four-year-old son because I thought he might find it confusing: With exception of 0 and 1, this book counts to 50 by twos. When I did give him this book, however, he recognized the numbers up to 30. I know he doesn't understand the counting-by-twos concept right now, that will come in time, but he does recognize the numbers. The facts in this book are interesting enough to engage the adult readers, and the text is well-written.
Underwater Counting is also a visually stunning book. The vibrant illustrations seem to swim off of the pages, and are so accurate, this book could be used for reference.
My son loves the aquarium and loves numbers -- this book is perfect.