The word of the LORD came to Jonah son of Amittai: “Go to the great city of Nineveh and preach against it, because its wickedness has come up before me.” But Jonah ran away from the LORD and headed for Tarshish. He went down to Joppa, where he found a ship bound for that port. After paying the fare, he went aboard and sailed for Tarshish to flee from the LORD. Jonah 1:1-3

God asked Jonah to go to Tarshish and preach to those in Nineveh who were enemies of the Hebrews. Those who lived in Ninevah were Assyrians, an evil group of people.

Jonah then ran away and got on a boat going the other way. However, a great storm struck the boat. Jonah told the crew that the storm happened because he was sinning and told them to throw him overboard.

“Pick me up and throw me into the sea,” he replied, “and it will become calm. I know that it is my fault that this great storm has come upon you.” Instead, the men did their best to row back to land. But they could not, for the sea grew even wilder than before. Then they cried to the LORD, “O LORD, please do not let us die for taking this man’s life. Do not hold us accountable for killing an innocent man, for you, O LORD, have done as you pleased.” Then they took Jonah and threw him overboard, and the raging sea grew calm. At this the men greatly feared the LORD, and they offered a sacrifice to the LORD and made vows to him. Jonah 1:14-16

Jonah was willingly thrown overboard (He did not jump).  It was an act of repentance because of his disobedience. 

And it was a sea, not a lake.