沖 (oki) means “open sea” | MotiTan Japanese Dictionary

More examples

漁船が沖で魚を捕っています。gyo sen ga oki de sakana o to tte i ma su

A fishing boat is catching fish offshore.

It refers to the part of the sea that lies outward from the coast rather than beside the shore.

外海gaikai: The open sea outside bays, harbors, and other sheltered waters.
沖合okiai: Sea located at a distance from the shore.

陸地や岸から離れた海域。riku chi ya kishi ka ra hana re ta kai iki

沖 (oki) is an old native Japanese word meaning the sea beyond the shore, but its earlier word history is not certain.

沖 (oki) appears in the place name 沖縄 (Okinawa), but 沖縄 is a proper name and does not mean "open sea" in ordinary Japanese.