I did really enjoy this one but not as much as the others. This book is one of those ones in a series that is an added extra in the middle. It's in Ina's perspective after Zeke has gone off with Chloe back to land and she and her grandfather are left in the ocean.
Often people read the little side-books because they don't want to miss something important later on in the story when it's mentioned and you're sitting with the book thinking: "what on Earth? When did this happen?" and so the reader tends to read them through fear of not understanding something.
I would say that in this book, although it is still really good, there isn't anything in it that you really need to know, nothing comes up about it in the further books so you don't need to worry.
It isn't overly exciting, it's one of those side-stories that isn't particularly interesting but you are still drawn to it anyway.
So yeah, still really good and you learn a little more about Ina and their grandfather but otherwise, there isn't anything in it that you absolutely have to know in order to understand different parts of the following books.