Malta
The main island of Malta offers one of the densest concentrations of dive sites in the Mediterranean, combining reef, cave and wreck diving around its coastline.
Visit Malta's official tourism site describes 'over 120 dive sites within minutes from each other', including the West Reef and Caves site. Malta has more purpose-sunk and naturally occurring wrecks per square kilometre than almost any other Mediterranean dive destination, including the Um El Faroud tanker (scuttled 1998), the Rozi wreck off Cirkewwa, and HMS Maori in Grand Harbour. Coordinates are Wikipedia's infobox coordinates for the island of Malta as a whole, not a single dive site. Source: visitmalta.com, Wikipedia (Malta island).