General:
* Sea: This is the most common synonym for ocean.
* Main: An archaic word for the sea.
* Deep: Refers to the ocean's depth.
* Vast: Emphasizes the ocean's size.
* Blue: Refers to the color of the ocean.
* Water: A general term for the ocean, but can be ambiguous.
More specific:
* Bight: A large curve in a coastline, like the Bay of Bengal.
* Gulf: A large inlet of the ocean, like the Gulf of Mexico.
* Sound: A narrow body of water, usually connecting a larger body of water to a bay or lagoon.
* Straits: A narrow passage of water connecting two larger bodies of water.
Poetic:
* The briny deep: A more poetic way to refer to the ocean.
* The watery expanse: Emphasizes the vastness of the ocean.
* The great unknown: Refers to the mysteries of the ocean.
The best word to use depends on what you're trying to convey.