Fish actually do know there's something outside of the pond. They may not be able to reach it for long, but some jump into the air, some spit water to shoot down insects, some even climb onto land, and many come to or near the surface where they can find and try to avoid becoming food.
On the surface it seems like a decent analogy, but when you look at the details and see how much interaction there really is between a pond and the outside world, it falls apart.
I think a better comparison would be that a pond is to the Earth as the Earth is to the solar system. We know that there's more outside of our little bubble, and we can have limited interactions with it, but we're not free to venture out into it at will. Unlike the author's supernatural realm though, we have plenty of evidence that this one exists.