Besides the regular ferry services between Honshū, Shikoku and the various islands, SKK (Seto Naikai-kisen; 082-253-1212; ticket office 7am-9pm) offers day cruises on the Inland Sea from Hiroshima. The trips are seasonal, and cruises with lunch/dinner cost from ¥5000/7500.
The Japan Travel Bureau (JTB) and other tour operators also run seasonal overnight cruises in the Inland Sea.
It’s possible to take a ferry through the Inland Sea from Kansai to Kyūshū. Unless you check the times carefully, though, you may end up going through the Inland Sea at night, and if you just travel through you won’t get a chance to taste the lifestyle on any of the islands. Ferry Sunflower (06-6572-5181; www.ferry-sunflower.com in Japanese; ticket office 9am-5pm) has two daily Osaka–Beppu ferries, and Diamond Ferry (078-857-9525; www.diamond-ferry.co.jp in Japanese; ticket office 9am-5pm) has two ferries a day between Kōbe and Oita on Kyūshū.