The Octonauts and the Sea of Shade 05/05/11
The Octonauts and the Sea of Shade by Meomi is the second book in the series. It's also the odd ball in the series in that it's not based on a real creature of phenomena of the sea.
In this book the Octonauts notice that the shadows have disappeared from everything. They decide to investigate and go into the sea of shade. The ruler of the sea of shade has decided he doesn't like his shadows being treated as second class citizens. The Octonauts have to negotiate with him and the shadows themselves to come to a solution.
The book is a good story about segregation and treating people fairly regardless of their job, appearance of position in life. But all the other books in the series deal specifically with ocean topics and this one doesn't. It's allegory and fantasy and doesn't lead naturally into science discussions.