if jonah just jumped off the ship, there would be no testimony for the others on the ship.
And they would not have seen who the true God is.
Certainly not the gods that they worshiped.
Jonah 1:5-6,9-10,12,16 KJV
Then the mariners were afraid, and cried every man unto his god, and cast forth the wares that were in the ship into the sea, to lighten it of them. But Jonah was gone down into the sides of the ship; and he lay, and was fast asleep.
[6] So the shipmaster came to him, and said unto him, What meanest thou, O sleeper? arise, call upon thy God, if so be that God will think upon us, that we perish not.
[9] And he said unto them, I am an Hebrew; and I fear the LORD, the God of heaven, which hath made the sea and the dry land.
[10] Then were the men exceedingly afraid, and said unto him, Why hast thou done this? For the men knew that he fled from the presence of the LORD, because he had told them.
[12] And he said unto them, Take me up, and cast me forth into the sea; so shall the sea be calm unto you: for I know that for my sake this great tempest is upon you.
[16] Then the men feared the LORD exceedingly, and offered a sacrifice unto the LORD, and made vows.