The oceans are the vast bodies of water that cover most of the earth. In English, different parts have names: the Pacific Ocean, Atlantic Ocean, Arctic Ocean, Southern (or Antarctic) Ocean and Indian Ocean, but in truth, it's all one ocean. You could say, "I crossed the Pacific Ocean when I flew from Los Angeles to Tokyo." But you wouldn't say "sea" here because you're referring to a specific ocean. A sea is any large body of salt water, including both oceans and saltwater lakes. Often, it is part of an ocean (for example, the Mediterranean Sea is part of the Atlantic Ocean, or the Caribbean Sea is part of the Atlantic Ocean). Often it is used as a synonym for the ocean "I spent seven days adrift at sea" would usually be taken to mean you were in the ocean. Usually you would say "at sea" or "on/in the ocean" but you wouldn't switch the prepositions.

Ocean and sea are synonyms. Meaning for most things, that either can be used. However this does not apply to proper names. "Pacific Ocean" should not be called "Pacific Sea", and "Sea of Japan" should not be called "Ocean of Japan".