I recall having to read this as part of my fourth grade science class.
As a book popularizing science, it's not very good. One learns precious little from this book about crabs or aquatic life. The book mentions Pagoo's molting and search of food, but that's where its level of "science" begins and ends.
However, as a literature story, it's also quite poor! There is no real conflict or excitement to the story. Instead, Pagoo goes through a banal series of parts of his life, which the author probably crossed off like so many check marks. It feels like a series of chores more than anything.
While there was nothing obscenely bad about the book, and I generally like titles that try to get kids interested in science, there was absolutely nothing good in it either. Even 2 out of 5 stars ("okay") is being highly generous.