The JR site mentions it specifically, highlighting the fact that the Seishun 18 ticket is valid for the ferry to the island.Getting there in a single day is a huge haul from Tokyo—but it is possible.
Down near Hiroshima, Miyajima is the place with the iconic red torii gate, the base of which is submerged at high tide.
You can share the tickets, too; a group of five people could each travel for one day, for example, or a couple could get two days of travel each (with one to spare).
The only catch is that you can’t ride limited express trains (tokkyu/特急) or the Shinkansen (bullet train).
You can buy the tickets at most JR stations, but only three times a year.
If you look at the official JR website, you’ll see that it says the tickets go on sale just twice a year—that’s out of date, and you can confirm as much on the Japanese page.