Well that just plain isn't the case. Up and down are relative directions, not absolutes, and as fish are unable to think like this, we (humans) are the only ones able to define "up" or "down".
Even for us they are relative, but generally speaking on earth they are defined either by gravity or by feet and head on a human body standing up. Either way, whatever way a fish swims, it's not upside down by our definitions. And however you chose to define it, we still class it by our definition, not by "what might we think if we were a fish capable of human levels of thought?".
And as to front/back... well that's theoretially described by the direction of the animal's normal movement. But regardless of that, again if you are defining a fish, it can only be defined by our grammar, not by their world.
that is, "back" would usually mean the location of the spine. Regarding how the fish perceives or thinks about its orientation I think this is impossible to say!
Yes they aren't identical situations, but you'll notice in your diagram that there is a downward force acting on the fish called "gravity", which is the point I was making - that gravity is acting on fish and humans regardless of what upward forces are also present.
Try to appreciate the sense in which his statement might be true before nitpicking. It's true that free-fall feels substantially different from floating underwater, or even lying on a soft surface (the last two are more similar subjectively than free fall - the force resisting gravity is spread over your body).
