Jonah was famously swallowed by a huge sea creature. Here is the KJV's description, Jonah 1:17:
Jesus speaks about this event. This is Matthew 12:40:
Now, as I trust everyone knows, whales are not fish. But this is just a KJV error. The Greek word in Matthew is ketos, which is a generic term that can apparently refer to any large sea creature. Most modern translations of Matthew read something like this:
Problem solved.
In addition, it's worth asking why we're so sure that whales are not fish. The answer is that whales are related to the other mammals. We humans, for example, have common ancestors with the whales. The most recent of these ancestors lived about 85 million years ago. And while mammals are also related to fish, this relationship is much more distant: the most recent common ancestors of mammals and fish lived about 400 million years ago.
Obviously a Creationist denies all this, which leaves it open for him to say that there's no good reason why whales can't be called fish. However, it was recognised by Linnaeus in the 18th Century (long before Darwin) that whales are mammals, on the basis of their anatomy. Today one can also compare whale DNA to the DNA of both mammals and fish, and yes, whales are indeed mammals.
But none of this is a problem, since it is not Jesus but the KJV that is in error.