The sea experiences periodic red tides caused by dense groupings of certain phytoplankton that result in the death of large numbers of fish.

A line running from Nagoya Saki (130°49'E) in Kyûsû through the islands of Uma Sima and Muture Simia (33°58',5N) to Murasaki Hana (34°01'N) in Honsyû].

Examples are the ayu, an amphidromous fish, horseshoe crab, finless porpoise, and great white shark, which has occasionally attacked people in the Seto Inland Sea.

From ancient times, the Seto Inland Sea served as a main transport line between its coastal areas, including what is today the Kansai region and Kyushu.

In the 12th century, Taira no Kiyomori planned to move the capital from Kyoto to the coastal village of Fukuhara (today Kobe) to promote trade between Japan and the Song dynasty of China.

The Seto Inland Sea also served many daimyōs in the western area of Japan as their route to and from Edo, to fulfill their obligations under sankin-kōtai.

The Seto Inland Sea was also part of the official Chosendentsushi route, bringing Korean emissaries to the shogunate.

Remarkable land transportation innovations include the San'yō Main Railroad Line in Honshū and the Yosan Main Railroad Line in Shikoku (both completed before World War II) and three series of bridges connecting Honshū and Shikoku (completed in the late 20th century).

Major cities with heavy industrial activity on the coast of the Seto Inland Sea include Osaka, Kobe, and Hiroshima.

Smaller scale manufacturing and industry can also be found in Kurashiki, Kure, Fukuyama, and Ube in Honshū, and Sakaide, Imabari, and Niihama in Shikoku.

Thanks to the moderate climate and beautiful landscape, fishing, agriculture, and tourism bring a lot of income to the area as well.

Historically, the Seto Inland Sea as transport line served four coastal areas: Kansai, Chūgoku, Shikoku, and eastern Kyūshū.

After Kobe port was founded in 1868 to serve foreign ships, the Seto Inland Sea became a major international waterway with connection to the Pacific.

Many short railroads were planned to connect a certain station of those two lines and a local seaport on the Seto Inland Sea, and some of them were actually built.

When the Great Seto Bridge was finished and began to serve the two coastal areas, that ferry line was abolished.

The Akinada Tobishima Kaido route connects seven of the western Geiyo Islands to mainland Honshu near Kure, Hiroshima.

Even before the country opened to foreigners in the middle of the 19th century, the sea's beauty was praised and introduced to the Western world by those who visited Japan, including Philipp Franz von Siebold, and after the country's opening, Ferdinand von Richthofen and Thomas Cook.

Hiroshima is the neighbor city to Itsukushima Shrine and another UNESCO World Heritage Site because of atomic bomb damage in 1945.

At the far eastern extremity, as the Sea meets the Pacific Ocean, are the Naruto whirlpools that can be reached by sight-seeing boats.

Here Richard Henry Brunton built one of his lighthouses that can still be seen, and the grave of Frank Toovey Lake, a young midshipman in his survey party, is on the island of Sanuki Hiroshima.

Some sites along the Seto Inland Sea were featured in eighth-century Japanese literature, both in prose and in verse, including Kojiki, Nihon Shoki, and Man'yōshū.

In medieval literature, because of the Genpei War, the Seto Inland Sea is one of the important backgrounds of The Tale of the Heike, particularly in its latter part.

In the Western world, Donald Richie wrote a literary nonfiction travelogue called The Inland Sea relating a journey along the sea, beginning from the East at Himeji and ending at Miyajima in the West, close to Hiroshima, going from island to island, exploring the landscape, meeting and discussing with local people, as well as musing on Japanese culture, the nature of travel and of identity, and his own personal sense of identity.

In 1991, filmmakers Lucille Carra and Brian Cotnoir produced a film version of Richie's book, which further explored the region through interviews and images photographed by Hiro Narita.