"While a "sea" is generally defined as a large lake that contains saltwater, or a specific portion of an ocean, the idiom "Sail the seven seas," is not so easily defined. "- 
- The North Sea between the British Isles and Scandinavia
- The English Channel between England and France
- The Red Sea between Arabia and Africa
Bodies of water that are called seas but don't join an ocean are in fact lakes with a different name.
- The Dead Sea is in fact a salt lake and much saltier than any ocean.
- The Caspian Sea is saltier than a typical freshwater lake but less salty than any ocean.