As you know, Japan is an island nation consisting 6852 islands and just 418 of them are inhabited. Then, nearly 15% of the islands are located in Setouchi inland sea, including famous Miyajima, Naoshima and Shodoshima islands.
Island in Japanese is SHIMA or JIMA, therefore Miyajima should be called just Miya island, but if talk to Japanese and ask them how to reach MIYA ISLAND, probably they would not understand and recongnize that Miya island is Miyajima.
By the way, Fukushima is a name of the prefecture and the prefectural capital, but Fukushima city is landlocked and also Fukushima prefecture has 13 islands but all of them are smaller than 1 km2.