Claws, by Michael Tougias
Life on Shipwreck Island is a bore for twelve-year-old Easterly Wind and her friends, Brian (age nine) and Kristin (age ten), until something strange happens just before the island’s annual Summer Celebration Dinner. A lobsterman goes missing while fishing off the island, and there are signs of foul play. The three children become obsessed with the mystery, and begin an investigation that leads them down a path of adventure.