With all due respect to Lewis, I don't completely buy his argument. By that reasoning, we could assert that the sun is a giant marshmallow that provides us light and heat through some incredibly miraculous provision of the Almighty. Or we could believe that that the sky is a dome on the other side of which is a giant reservoir of water (Genesis 1), and that God makes it rain by opening floodgates (Genesis 7). We also should accept that the solar system is geocentric (Joshua 10:12-13), that the sea is contained by doors (Job 38:8), or that God has warehouses full of snow and hail (Job 38:22).
But of course we don't. God gave us eyes, ears, and hands, and minds to study the world that he created, and our study of the world has shown us that the solar system is heliocentric, and that snow, hail, and rain are formed by natural, observable processes that involve an endless recycling of the water on the earth.
And in all the cataloging we have done of the creatures of the sea, we've yet to find a fish (or other large sea creature that falls under the Hebrew term for "animal that lives in water") with a gullet large enough to swallow a man and keep him alive for three days so that he can be coughed up on dry land. Maybe there's something out there we haven't found yet, but I doubt it. Not on that scale.
And given the Hebraic love for storytelling to communicate great truths, and given that the notion of taking Scripture at face value in the manner under discussion is a relatively recent phenomenon -- i.e., early 20th century Christian -- I don't have any qualms in saying it's a work of divinely inspired fiction.