18 The prophet is indeed ashamed, foreseeing what the issue would be, and conjecturing that what he had prophesied, would remain unaccomplished; God however is not ashamed, but is desirous of one thing only, viz. the salvation of men, and corrects His own servant. For when he had entered the ship, He straightway there raised a boisterous sea; in order that you might know that where sin is, there is a tempest; where there is disobedience, there is the swelling of the waves. The city was shaken because of the sins of the Ninevites; and the ship was shaken because of the disobedience of the prophet.
The sailors therefore threw Jonah in the deep, and the ship was preserved. Let us then drown our sins, and our city will assuredly be safe! Flight will certainly be no advantage to us; for it did not profit him; on the contrary, it did him injury. He fled from the land indeed, but he fled not from the wrath of God; he fled from the land, but he brought the tempest after him on the sea; and so far was he from obtaining any benefit by his flight, that he plunged those also who received him into the extremest peril.
And while he sat sailing in the ship, although the sailors, the pilots, and all the necessary apparatus of the ship were there present, he was placed in the utmost danger. After, however, having been thrown out into the deep, and having put away his sin by means of the punishment, he had been conveyed into that unstable vessel, I mean, the whale's belly, he enjoyed great security. This was for the purpose of teaching you, that as no ship can be of any use to him who is living in sin, so him who has put away his sin, the sea cannot drown, nor monsters destroy.
Of a truth, the waves received, but they did not suffocate him. The whale received him, but did not destroy him; but both the animal and the element gave back to God unhurt that, with which they were entrusted; and by all these things the prophet was taught to be humane and merciful; and not to be more cruel than wild beasts, or thoughtless sailors, or unruly waves. For even the sailors did not immediately at first give him up, but after much compulsion; and the sea and the monster guarded him with great kindness; all these things being under God's direction.
Source: Homilies on the Statues (New Advent)