Young Captain Nemo

Gabriel Nemo and his family have spent their lives trying to undo the legacy of their ancestor, Captain Nemo, who wasn't exactly a hero.  His parents live in a hidden lab at the bottom of the ocean, his sister uses her large imposing ship to impose her own morality on ocean vessels, and Gabriel goes to middle school.

His parents sent him to the California coast where his lives alone in a house that is really a secret lair for his state of the art Nemotech submarine Obsure.  He's supposed to be acting as a sort of ambassador with the world, but really he goes to school and performs daring sea rescues with the help of his two best friends, Peter and Misty.

When his sister Nerissa comes across a strange new creature, she enlists Gabriel's help.  No one has ever seen anything like the Lodgers.  They are giant mollusks, so large they are using wrecked ships and downed airplanes as shells.  And they also sharp teeth and long tentacles they use to secure new "shells" whether they are currently occupied or not.  And the Lodgers have run afoul of the U.S. Navy.

Now Gabriel and his family have to find a way to save the Lodgers before time runs out.  But it won't be easy to save these massive creatures when they don't know who their friends are, and every encounter is potentially deadly!

This is the first book in a planned series by Jason Henderson, and, yes, you will have to suspend your disbelief A LOT, but this is a fast-paced, high action, thriller.  I can imagine multiple kids who will enjoy this book.  It has sea monsters and a standoff with the Navy!  Recommended.