The point is that in principle through convergent evolution our descendents could develope something like a gill, scales swimming habilites and other traits that we commonly associated with fish.
Hence humans can evolve in to something that we would call fish
You obviously agree with this point but you won't admit it. But rather you will make a strawman and refute that strawman
Yes. Organisms evolve traits facilitating their environmental fit. Fish models -- and these vary quite a bit -- fit well. A non-fish moving into their environmental niches would be expected to evolve some of their features and capabilities.
But mammals returning to the sea would be starting from a different place than the progenitors of fish. They could not retrace the path that produced fish. They would be starting with a different anatomy and physiology.
Remember, evolution works with what already exists. It tweaks existing structures and biochemistry to deal with novel environmental situations. It cannot start from scratch to engineer an ideal fit. Organisms are Jerry-rigged.
Gills? What structure would be altered to produce gills? True, lungs can "breathe" hyperoxygenated water for a short time, but not easily or permanently, and major problems arise.
No existing air-breathing mammal or bird that's returned to the water has developed this capacity. They remain air breathers who've tweaked their A&P to facilitate breath-holding, surface air-gulping, and efficient oxygen utilization. In fact, fish altering swim bladders to utilize air for gas exchange have been more successful at developing "lungs" than any mammal has developing gills.
Scales? Why would a water-dweller need scales? Not all fish have scales. Scales developed from protective denticles or plates, not to directly facilitate water-dwelling.
Swimming abilities? Not a problem. Mammals and birds easily alter existing extremities to facilitate swimming all the time.
A mammal would adapt to an aquatic environment as a swimming mammal, not in the ways fish have evolved their fit.