And human don't know they see the fish upside down. To a fish, the ground is the water level. The fish belly is actually their back.
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.
No, you english speaking guys are the only ones able to define "up" or "down".
http://en.wikipedia.org/wiki/Posterior_%28anatomy%29#Anterio...
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!
In the second case, gravity acts on you even when you are going up, in fact it is what slows you down and then makes you start to fall.