Was Jonah thrown into the sea by the sailors or by God?

Both, and the book itself shows how. The sailors threw Jonah overboard with their own hands (1:15), and only after he had told them to (1:12) and they had rowed hard to reach land instead (1:13). But the whole crisis was God’s doing from the start. It was the Lord who “sent out a great wind into the sea” (1:4), and the lot they cast “fell upon Jonah” (1:7). The sailors knew this. As they lifted him they prayed, “thou, O Lord, hast done as it pleased thee” (1:14). They saw themselves as carrying out God’s will, not acting on their own.

Jonah’s prayer in chapter 2 is a psalm of thanksgiving, and like many psalms it speaks of what God has done through human and natural means. He borrows the words of an earlier psalm (Psalm 42:7) and calls the waves “thy billows and thy waves” (2:3) for the same reason: God had brought him into the deep, whoever’s hands were used. To say that someone did a thing, and that God did it through him, is a common way of speaking in Scripture. Joseph told his brothers, “it was not you that sent me hither, but God” (Genesis 45:8), though they had sold him. Gill makes the point on Jonah 2:3: although the sailors threw him in, Jonah gives the act to the Lord, who had sent the storm, settled the lot and overcome the men’s reluctance. The two statements describe one act from two sides.