According to our encyclopedia the words are synonymous.
In addition, the word "sea" can also be used to describe a subdivision of an ocean that is more or less marked off by land boundries. That's the case with the North Sea, Caribbean Sea, and Yellow Sea. It is inaccurately used to describe large, land-locked salt-water lakes like the Caspian Sea, Dead Sea, and Aral Sea.
—The Fact Monster