Billy and Sheena are staying with their uncle "Dr. D." over the summer on his ship. So they end up like, looking for mermaids. Or something.
Billy manages to be continually stupid, all the way up until the end. Going out for a swim, by yourself, in the early morning, in the ocean, is not a good idea! But he never learns.
Sheena is the obligatory annoying little sister who pinches Billy underwater. Yawn.
This book is not about sea monsters. There is a shark (which Billy somehow manages to evade for five minutes), mermaids, and Bad Guys, but the sea monster only makes two appearances, and very brief ones at that.
There is a moderately effective drowning scene, but it doesn't make up for the utter dullness of the rest of the book.
Oh: and at one point a mermaid cries underwater.
Tears roll down her cheeks.
Underwater.