Origin

The character combines the water radical on the left, which marks this as a word related to water or the sea, with the character 中 (chuu) on the right, which serves as a sound hint to indicate pronunciation. The shape essentially tells you "something related to water that sounds like chuu." The meaning "open sea" likely developed because the character describes water that is far from shore and rises up high, capturing both the vastness and the swelling motion of the offing.