Escape to the Moon islands - Mardi McConnochie (Quest of the Sunfish #1)

Goodreads 3.96

Forty years after a devastating flood changed the face of the earth, new consequences and challenges are still surfacing.

Twins Will and Annalie thought the hardest part about this year was going to be their separation when bookish Annalie began life at a prestigious Admiralty-run boarding school and avid sailor Will stayed behind in the flood-damaged slums. But that was before the Admiralty raided their father’s workshop. Before they sent a questioner to threaten Annalie at school. Before their father disappeared, leaving a single coded clue to his destination. Desperate for answers and to find their father, who is the only other family they know, the twins set out in the family’s small sailboat. But though they are both experienced sailors, they have no idea what dangers the sea has in store.