I know you can buy JR Rail passes for travel by train all over Japan.
But Japan also has many islands and I assume quite a few of them are serviced by ferries.
What I'm wondering is whether there is anything resembling a "ferry network" where you can buy a single ticket or pass that covers a range of ferries between different islands.
If not for the entire country, then at least for particular areas with many islands, such as Okinawa. Or even for any group of three, four, or five islands.
(If there are several such passes each serving small groups of islands this may be changed to a Community Wiki question.)