While the story of Jonah and the whale (or sea monster or fish or whatever) has long been a source of ammunition and ammusement for those who want to ridicule the Bible, this is truly one of the most doofus objections to the Bible that I've ever heard.
The argument is basically that the story is ridiculous; that a whale couldn't swallow a man, and if it did the man would surely drown.
To which the proper response should be: "Well... of course!"
This story is about what is is called a "miracle," and - do I really need to explain this? - miracles violate the basic laws of nature. If they didn't they wouldn't be called "miracles."
If the Bible said Jonah was swallowed by a minnow, I wouldn't have the slightest philosophical problem with that. God is quite capable of putting a man into a minnow. In fact, scientists - whom the critics presumably believe - say that the whole universe was once smaller than a pinprick, so if the universe was once so small that it could get lost in your empty back pocket, what's so hard about fitting a man into a big fish?
Note: There has been some discussion about whether Jonah was kept alive in the sea creature or was brought back to life after his underwater trip, but either way we're talking about a miracle.