The Shiwaku Islands or Shiwaku-jima are an archipelago in the Seto Inland Sea, between the larger Japanese islands of Honshu and Shikoku. The group is situated between Okayama Prefecture and Kagawa Prefecture in the western Bisan Seto and consists of 28 islands of various sizes. On the Okayama side lie the Kasaoka Islands. The name deriv…The Shiwaku Islands or Shiwaku-jima are an archipelago in the Seto Inland Sea, between the larger Japanese islands of Honshu and Shikoku. The group is situated between Okayama Prefecture and Kagawa Prefecture in the western Bisan Seto and consists of 28 islands of various sizes. On the Okayama side lie the Kasaoka Islands. The name derives from shioyaku or shiowaku both meaning boiling seawater to get salt. However, the name may also refer to rough water created by the confluence of tides that mix around the islands.