I’ve never heard this version with a duck floating past. Wallace’s original quote (as far as I can tell) is this screenshot I took several years ago:

The duck puts a spin on the parable that the old fish doesn’t. Presumably the old fish was once a young fish himself and somehow came to an understanding of the reality surrounding him, suggesting the young fish might do likewise because their nature is the same.
A duck is a completely different animal (pun intended) capable of flight, among other things. By its nature, the duck’s experience of water is completely different from the young fish. With the occasional exception of a deep dive, his *surface/superficial knowledge of water has him necessarily looking down on those experiencing water from within most of the time. It’s arguable who ‘understands’ water better.
In any case, it makes one think. I’m sure other astute readers could come up with interesting juxtapositions between these two versions of the parable.
* ‘surface’ (as an adjective) and ‘superficial’ have the same translation in Russian: поверхностный. Both are appropriate to what I meant here. I love words and parables with nuanced multiple meanings.
