You are saying that a fish, that could think like a human, would still be a fish ?
We call something "a fish" or "not a fish" simply based on whether it would be useful, for our communication, to do so.
We name things based on utility. If the utility picture changes, as it does with a fish who can think like a human (and therefore might be able to kill you in your sleep by splashing water on your computer and starting an electrical fire), we no longer would likely feel that the word "fish" evokes the most useful imagery for equipping someone to deal with that creature when we communicate about it. We might feel compelled to qualify it as a "superintelligent fish" or even a "human-fish." Whatever is most useful for getting the point across that you don't want to underestimate its intelligence.
Once you understand that we name things based on (methodologically individual) utility, many paradoxes are resolved. Here are two examples.
Paradox of the Heap: How many grains of sand does it take to make a heap?
Utility phrasing makes it easy. A "heap" simply means a point where you yourself find no utility in trying to keep track of individual grains in the set, either because you're unable to easily count them or because it doesn't matter to you. "Meh, it's just a heap." The answer will differ depending on the person and the context. It is no set number; it's simply when you look over and stop caring about the individuated quantity. That is why this has the appearance of a paradox and why Wikipedia doesn't even mention this obvious and fully satisfying resolution. The fundamental error in Wikipedia's presentation is to consider what a heap "really is," rather than what the term "heap" can usefully mean for each person and context, even though it is self-evident that this is how language works.
Ship of Theseus Paradox:
"The ship wherein Theseus and the youth of Athens returned from Crete had thirty oars, and was preserved by the Athenians down even to the time of Demetrius Phalereus, for they took away the old planks as they decayed, putting in new and stronger timber in their places, in so much that this ship became a standing example among the philosophers, for the logical question of things that grow; one side holding that the ship remained the same, and the other contending that it was not the same."
—Plutarch, Theseus
Plutarch thus questions whether the ship would remain the same if it were entirely replaced, piece by piece. Centuries later, the philosopher Thomas Hobbes introduced a further puzzle, wondering what would happen if the original planks were gathered up after they were replaced, and used to build a second ship. Hobbes asked which ship, if either, would be considered the original Ship of Theseus.
This is also easily and satisfyingly, though again un-excitingly, resolved by utility phrasing. "Ship of Theseus" is just a name we assign for utility purposes, basically to make life easier in our communications with ourselves and others. The name evokes certain associations for certain people, and based on that we will - in our communication efforts - call something "the Ship of Theseus" or "the original Ship of Theseus" whenever we believe that set of words will call up the most useful associations in the listener, to have them best understand our intent.
There is no such thing as a fully objective definition of the term "Ship of Theseus"; it always depends on what you're attempting to communicate to whom, and what you/they actually care about in the present context.
For example, if it matters to you that the ship was touched by Athenian hands, it wouldn't be useful to you to refer to it as the "Ship of Theseus" if all the parts had been replaced by non-Athenians. But if you simply cared about the way the ship looked and what it could do, because it has a unique shape and navicability compared with other ships, it would be useful in your mind to refer to it as the "Ship of Theseus" even if its parts had all been replaced for functionally and visually identical ones.
Once again it comes down to each person's utility of calling it one thing or another in each context. We will call the a second ship built in the image of the first a "replica" if we are speaking in a context of attributing credit for its design and original building, but simply call it "a Ship of Theseus" if we only care about its function and looks in this context, and we'll call it "
the Ship of Theseus" even if it is not the original if the original has been destroyed and all we care about is the form and function, such as to answer a practical question like, "Can the Ship of Theseus sale to Minoa?"
To repeat the above point, the key error is in considering what the Ship of Theseus "really is," rather than what the term "Ship of Theseus" can usefully mean for each person and context. Even though it is self-evident that this is how language works in the first place, people are nevertheless highly prone to this kind of error (the reasons have to do with tribal instincts).