The island of Elba, famous for Napoleon's 10-month exile, is the largest island of the Tuscan archipelago. It's probably also the easiest to navigate as a tourist, with lots of shops, restaurants, hotels, sandy beaches, and of course: great SCUBA diving.
The local company Diving in Elba has SCUBA classes and excursions, including a "Baby School" for kids 8 and up.
Or contact Scuba's Angels, a team of female diving instructors, fluent in English and based out of Turin, who organize single- or multiple-day diving trips.
If you're hiring a boat and just want to know where to dive, this website has details on great dives around Elba.
The Best of The Rest
Rent a car on the island to visit some of the 150 beaches (!) or the pretty towns of Capoliveri, Portoazzuro, or Marina di Campo.
The island's food is Tuscan – with an emphasis on seafood of course – and Elba produces more types of local wine than I have room to list.
Elba is also interesting for history- and archaeology-lovers, having been inhabited by the Etruscans, Romans, and Spanish; and fought over by the English, Germans and French because of its strategic position.
With such a wide variety of things to do and so easily accessible, it's no surprise that Elba gets booked solid during the summer months, so plan accordingly.
The ferry to Elba's Porto Ferraio runs frequently and takes just 1 hour from the mainland port of Piombino.