Any large collection of water, more or less enclosed; hence the expression “molten sea,” meaning the great brazen vessel which stood in Solomon's temple (2 Chronicles iv. 5, and 1 Kings vii. 26). We have also the Mediterranean Sea, the Black Sea, the White Sea, the Red Sea, the Sea of Galilee, the Dead Sea, etc.; and even the Nile, the Euphrates, and the Tigris are sometimes called seas by the prophets. The world of water is the ocean. (Anglo-Saxon, sae.)
The Old Man of the sea
(Arabian Nights). A creature encountered by Sinbad the Sailor in his fifth voyage. This terrible Old Man contrived to get on the back of Sinbad, and would neither dismount again nor could he be shaken off. At last Sinbad gave him some wine to drink, which so intoxicated him that he relaxed his grip, and Sinbad made his escape.
At sea. Quite at sea.
Wide of the mark; quite wrong; like a person in the open ocean without compass or chart.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894