*The* ocean, is the large interconnected patch of water on earth. (Basically all water not on land, so not the rivers, lakes, …)
*An* ocean is used to refer to a part of *the* ocean. (Pacific, Atlantic, …)
*A* sea is a large body of salty water, so it could also be a lake, etc…
*The* sea is the same as *the* ocean.
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