It is not about a fish.
It is all about YHWH.
The opening chapter of Jonah is straightforward, but it also includes strong storytelling and interesting narrative devices. (Granted, most of these become lost in English translations . . . but there still remains good theological discussion within the story.)
One particular thought which jumped out at me today regards the nature of fear among the sailors. Already, the story has demonstrated that Jonah is going to be God's instrument of salvation even if he is heading in the completely opposite direction. This will be a theme throughout, and this opening scene foreshadows the much larger repentance which will come at Nineveh. When the storm rises up, the sailors are afraid for their lives (1:5), even though Jonah is sleeping in the bottom part of the ship. Once he confesses that this is YHWH's response to his disobedience, then the (pagan) sailors are more afraid because of an angry deity - one who is obviously powerful (1:10). They are gripped by fear, which is understandable and expected.
However, once the sailors comply with Jonah and throw him overboard, the sailors retain their fear - now transformed. They now consider YHWH differently and offer sacrifices to him (1:16). This sounds quite good, but does not possibly end the discussion on the topic. We must remember that these are pagan sailors from the Ancient Near East, and probably do not turn-and-convert to YHWH like they were at a Billy Graham crusade. Most of these men probably continued on in their polytheistic worldview, now adding YHWH to the mix. At best they might have considered that this God of Jonah was among the most powerful deities, but it is difficult to assume that they turned away from other gods.
So why then does the text mention their sacrifice? This is a Jewish story with a Jewish audience who would have understood proper sacrifice to YHWH as a very Jewish one. Would the storytellers have any reason to assume one way or another about this conversion? Or is it the opportunity for YHWH to make a powerful statement into the lives of these men, to the life-changing acceptance of him while others continue on as though nothing happened?
I suppose that if I were pressed on this, I would say that it is YHWH dealing with his prophet in full view of these sailors. Some of these men will concede their lives over to him, while others will not. It would be foolish to think that this was God's only way of working in the lives of these sailors, but this was not particularly about them. So it become collateral - not negatively but positively. The collateral damage here is their fear - once a fear of death, but now a fear of YHWH. And if he is the giver of life, it is not so much that they are afraid of life more than death . . . but of the one who stands behind it.
And there is no healthier fear in all of the world. For it is to these that YHWH himself promises no need to fear. It proves out that, as Brennan Manning has once said, "If you don't have to be afraid of God, you don't have to be afraid of anything."