The fish are swimming because they are in water, that is what is actually happening. If they do not know they are in water, they ascribe some other meaning as to why they are moving their bodies (swimming).

My interpretation is that the lack of realization of the young fish that they are swimming in water is analogous to a lack of wisdom, that people tend to not realize that their perceptions of reality (that they consider to be reality itself) are actually just a clever illusion constructed by the subconscious mind, and are based on all sorts of inherited cultural and experiential axioms.

Interestingly, people tend to be very good at identifying lack of wisdom in other people (particularly the members of their outgroups), but are terrible at identifying it in themselves - the classic example is people laughing at the stupidity of Trump supporters, anti-vaxxers, etc.

However, smart people are also very vulnerable to the very same phenomenon, I think it can be seen in this thread (and you'd be hard pressed to find a group that takes thinking more seriously than Rationalists):

https://www.reddit.com/r/slatestarcodex/comments/qz596o/why_...

Another lovely articulation of the same general idea:

Think you of the fact that a deaf person cannot hear. Then, what deafness may we not all possess? What senses do we lack that we cannot see and cannot hear another world all around us?

- Frank Herbert