That's one thing, but not the only thing. I suppose it's not impossible for a man to survive in the belly of a fish for three days, but that's not the main point of the Jonah narrative.
The big fish is a metaphorical vehicle for returning the reluctant prophet back to the Ninevites so that they may repent. Jonah's three days prefigures Christ's three days in the tomb.
I'm going to go the opposite way on this one.
Certainly the big fish means more than an odd episode contrary to man's typical experiences with aquatic life. I just wondered if the reticence in regarding the episode as historical is due to the notion it is impossible for such a thing to happen. As for myself, I see nothing in the text that would prevent one from understanding it as literally true, for which reason its lesson applies to reality at all levels, including the physical.