Malta
Malta's smallest inhabited island, Comino is a distinct diving area centred on the Blue Lagoon, nearby caves, and the P31 wreck.
Wikipedia describes the Blue Lagoon between Comino and Cominotto as having transparent cyan waters and being popular with divers and snorkelers. Visit Malta's official tourism site lists Lantern Point and the P31 Wreck Site as dedicated dive sites on Comino. Dive-operator sources additionally describe the Santa Marija Caves on Comino's north side, though this specific name is not corroborated on Wikipedia. Source: Wikipedia (Comino), visitmalta.com, blueholediving.com, maltadives.com.