Mizushima Sea, Sea passage between Okayama and Kagawa Prefectures, Japan.

Mizushima Sea (水島灘) is the part of the eastern Seto Inland Sea enclosed by Honshu, the Shiwaku and Kasaoka island groups and the Kojima Peninsula, bordering Bisan Seto and the Bingo Sea. The Kasaoka Islands lie within it, and the Kasaoka Bay inlet is a known horseshoe crab habitat. The name was given to Kurashiki's Mizushima district.

The waterway became critical for maritime trade during Japan's industrialization period, enabling the growth of Mizushima Port and surrounding commerce. This shipping route supported regional economic development as industries expanded along its shores.

Local fishing communities maintain traditional fishing methods in the Mizushima Sea, particularly around the Kasaoka Islands group of thirty-one distinct formations.

The sea is known as one of the Seto Inland Sea's most dangerous stretches for boats, so small craft should watch conditions closely. The coast is exposed to typhoon storm surges; Typhoon No. 16 in 2004 caused heavy damage here. Shimotsui, an old port town on the Okayama side, is reached by road and is known for octopus dishes and as a main shako shrimp landing port in the prefecture.

The sea is known as one of the Seto Inland Sea's most dangerous stretches for ships, and wrecks were long common. Sailors' ghosts were said to ask passing boats for a ladle, then sink them with seawater; the islets Ōbigyōi-jima and Kobigyōi-jima in the Mizushima islands are named after this legend.