Refers to the vast, open waters of the ocean, often contrasted with coastal or inland seas.
Ships that go on ocean voyages need thorough preparation.
この魚は外洋に生息している。
This fish lives in the open sea.
It's dangerous for a small fishing boat to head out into the open sea.
This ship is designed to be able to sail across the open sea.
Tuna are known as fish of the open sea.