Amami-Oshima Island, Kagoshima
Amami-Oshima Island, lying in the sea south of Kagoshima, is situated in the middle of the Amami Islands, which consist of eight islands, and ranks as the second largest remote island in Japan, after Sado-ga-shima Island in Niigata Prefecture. The sea around the islands, designated as Amami-gunto Quasi-National Park, is blessed with transparent waters, bright colored coral reefs and tropical fish, and is known as a fantastic diving spot. The northern part of the island, where Amami Airport is located, is a popular resort spot for divers that is dotted with white sand beaches and shores boasting of beautiful coral reefs.
More info: https://www.jnto.go.jp/eng/location/regional/kagoshima/amamioshima.html