Astoundingly, the sailors are reluctant rather than eager to get rid of their "Jonah," their source of bad luck. They don't make kaparot with him, spinning him over their heads and chanting "This is my atonement, this is my compensation, this is my redemption" before tossing him into the drink. They fear blood guilt more than they fear the storm, and try vainly to row to land. What gives the sailors their courage and their compassion? We're not told.
In any case, they do act out of courage and compassion. In the book that bears his name, only Yonah is named, and only Yonah acts badly: the four other groups of characters—the captain, the sailors, the people of Nineveh, and their king—all seem to be eager to do the right thing.
Yonah is cast overboard, and swallowed by a fish. From inside the belly of the beast Yonah offers, not a prayer to be saved, or a declaration of repentance, but a hymn of thanksgiving, as if his salvation were already an accomplished fact. Is he gaining insight and foresight? Or is he simply relieved to have regressed to the womb? We're not told.
Yonah is told again to go to Nineveh, and this time he complies. He makes what is undoubtedly the most efficient prophecy on record, if we measure prophetic efficiency in units of behavior change effected per word spoken. Yonah's prophecy, in Hebrew, is only five words long: "Yet forty days, and Nineveh will be destroyed." ("And Nineveh" and "will be destroyed" require only one Hebrew word each.)
In the face of this unadorned warning, the Ninevites, like the sailors, behave astonishingly well. They respond at once, proclaiming a fast. Why? How? We're not told. But the text makes it clear that the action comes up from the people and not down from their ruler. The King of Nineveh acts only after "the word reaches him" of what the people have already done.
Again, we're not told what sort of evil will otherwise bring down destruction on the city. The royal proclamation simply tells the people to turn aside from their "wicked ways" and the hechamas—variously translated "violence" or "unjust gain"—that is "in their hands."
Perhaps it is precisely the spare and unspecific nature of the prophecy that gives it such power. Say to someone, anyone, you, me: "You know, you really ought to cut that out" and he will know what you're talking about, even if you don't.
In any case, the people turn away (shavu) (a word which shares its root with t'shuvah, "penitence") from their "evil course" and God backs off: Elohim va-yinachem, which could be translated either "God repented of" (His intention to destroy the city) or "God took pity on" (the city He intended to destroy). The root is nachem: "mercy." Elohim turns aside from His intention to destroy Nineveh, that great city.
Yonah is angry that Nineveh is not destroyed. Why? We're not told. Presumably, because he feels that he has been made to look like a fool: he predicted disaster, and no disaster happened. Instead of rejoicing that his prophecy averted destruction, thus vindicating his insight, Yonah mourns that destruction was averted, thus casting doubt on his foresight.