Then the men were even more afraid and said to him [Jonah], “What is this you’ve done?” The men knew he was fleeing from the LORD’s presence, because he had told them. So they said to him, “What should we do to you to calm this sea that’s against us?” For the sea was getting worse and worse (Jonah 1:10-11 HCSB).
It’s interesting that these pagan sailors (“each cried out to his god” [1:5]) seemed to understand the necessity of personal propitiation with the gods. Jonah had made clear to them he was running away from Yahweh, Maker of Sea and Dry Land. They probed Jonah for what propitiatory rituals Yahweh required so Jonah could be consumed in His wrath but not take them down with him (literally, too, in this case).
“What should we do to you to calm this sea that’s against us?” They understood that Jonah had sinned, and something had to be done to him to atone for that sin. They were also guilty as those who aided and abetted his sin; he had told them, which made them complicit in his sin. Not only do they understand penal atonement, but they also understand penal substitutionary atonement: if Jonah absorbs the wrath of Yahweh the Sea-Maker, then they will be spared.
Jesus tells the Pharisees that Jonah serves as a sign for them: just as Jonah was in the belly of the fish for three days and nights, so too would the Son of Man be in the belly of the earth. Just as Jonah was brought back from the dead, so too Jesus rose from the grave for our justification.