I can understand "島" being used in 硫黄島 (Iwo Jima/Iwo To, literally "Sulphur island"), because it is an island, but why is it used in 福島市 (Fukushima city, literally "Good fortune island") and 広島市 (Hiroshima city, literally "Wide island")?
1 Answer
In short, that is because "island" is not the only meaning of 「島/しま/シマ」.
Besides "island", it can mean "settlement", "arable land by a river", "isolated area", "territory", "turf", "sandbank", etc. Even each section of a supermarket or any sizable store is called 「シマ」. So, it does not have to be sea water that surrounds a 「島/しま/シマ」.
「[福島]{ふくしま}」 was largely lakes and marshes at least as late as the 16th century according to chimei-allguide. So it was a kind of an island surrounded by fresh water.
「[広島]{ひろしま}」 is said to have started from a delta according to the same source as above. So, like Fukushima, it was like an island in a river if not the ocean.