In everyday language, the two are sometimes used interchangeably but there is a difference. Seas are smaller than oceans and are usually located where the land and ocean meet. Typically, seas are partially enclosed by land. There are only five oceans and more than fifty seas on Earth. I hope this answer helps!

Yes, oceans are enormous, and seas slightly less so - according to a US govt website: "Historically, there are four named oceans: the Atlantic, Pacific, Indian, and Arctic. However, most countries - including the United States - now recognize the Southern (Antarctic) as the fifth ocean. The Pacific, Atlantic, and Indian are the most commonly known."

Lots of good answers here. Here's another way of thinking about it. If you're referring generally to the large connected body of water on earth, they are pretty much interchangeable. I swam in the sea/ocean. We like to fish in the sea/ocean. The ship sails across the sea/ocean. If you are going to get more specific and geographical you need to specify which one: The Pacific Ocean. The Adriatic Sea. The Arctic Ocean. The Philipine Sea. etc.

One is 大海, one is 海洋 中学英语内容呢

There are only a handful of oceans. They have big waves and tides. They are enormous, like continents. The continents together with the oceans are the major entities that cover the earth. "Sea" is a more general term. Every ocean is a sea, but only very few seas are oceans. "Sea" can apply to almost any body of water. Even if you spent a day in a boat on a little lake, you could say that you had spent the day "at sea".