A sea is a body of water but not as large as an ocean. Could be used colloquially as synonymous of ocean. Also large lakes could be referred to as inland seas.
Some of the most important seas in the world are: The Gulf of Mexico, The Caribbean Sea, The Gulf of California, The North Sea, The Baltic Sea, The Mediterranean Sea, The Adriatic Sea, The Red Sea, The Arabian Sea, The Persian Gulf, The Dead Sea, The Black Sea, The Sea of Japan and The Philippine Sea.
“The fishermen know that the sea is dangerous and the storm terrible, but they have never found these dangers sufficient reason for remaining ashore” Vincent van Gogh.
Seas by ocean 
- After the Sea-Ship Walt Whitman (1819–1892). Leaves of Grass. 1900.