This is not your parents Kratt Brothers. Or more likely, your Kratt Brothers.
Chris and Martin Kratt took their love of animals and nature and turned it into various television shows and books. Here in the Wild Kratts (based on one television series) the brothers, in their cartoon style illustrations, created a level two reader about the critters of the sea.
Very basic information can be found in Wild Sea Creatures: Sharks, Whales and Dolphins. The aged aimed for (like many level twos, but not all as have mentioned before, each publisher has their own system) is for the middle beginning reader. The text is not hard, but it is not easy either. Assistance might be required for the solo reader. It does not lend itself to an easy read-aloud but can be a one-on-one read. If your child is a fan of sea creatures, but is not looking for something too heavy, this might be something to try.
Of course, if the reader is not a fan of the Kratt Brothers Wild Kratt show, the illustrations might be a bit off putting as they are surreal and fantastical (the guys submarines look like the animal they are talking about, they put themselves in situations that are unrealistic). The colors are deep and bold reflecting the oceans image and creature talking about. Which are not always just the “usual suspects” but also include things like Yeti Crabs.
The nice thing is if your child is a fan of Martin and Chris, there are various formats you can get: early readers, slightly older readers, the 8×8 sized longer paperback or even a Golden Book size.