There are two fundamentally different ways to remove a watermark, and tools rarely tell you which one they are doing. The difference decides whether you get your original pixels back or a convincing invention.
Alpha removal: reversing the composite
A visible watermark is applied by blending: each covered pixel becomes a mix of the original and the mark, in known proportions. If you know the mark's shape and how strongly it was applied, that blend can be run backwards — subtract the mark's contribution, scale what remains, and you have the original pixel back.
When it applies, this is not a reconstruction at all. It is arithmetic, and the result is exact. It needs a known mark with a known opacity, which is why it works so well for a specific generator's logo and not at all for an arbitrary watermark someone stamped on a photo.
Its failure mode is saturation: where the mark was opaque enough to have pushed pixels all the way to white, the original information was genuinely destroyed and no arithmetic recovers it.
Inpainting: inventing what fits
The general case is a mark you know nothing about. Here a tool has to invent replacement content consistent with the surroundings — either by copying texture from nearby, or by asking a model what probably belongs there.
Good inpainting is remarkable on smooth or repetitive backgrounds. It is unreliable on anything specific, because "plausible" and "correct" separate the moment the answer is not implied by the surroundings. A model asked to reconstruct a covered eye will produce an eye — just not that person's.
The blur trap
Weak tools do neither and simply blur or clone-stamp the region. It looks acceptable at thumbnail size and obviously wrong at full size: a soft rectangle where the rest of the photo has grain. If a result looks smoother than its surroundings, that is what happened.
Why the seam matters more than the fill
Even a perfect fill looks pasted if its brightness does not match where it landed, because donor pixels come from a differently lit part of the image. The fix is to keep only the replacement's texture and rebuild its brightness from the rim of the hole. That step is the entire difference between "pasted on" and "repaired", and it is what the patch tool does on release.
Which you want
For a known generator mark, alpha removal — exact where it applies. For everything else, guided patching, where you choose the source rather than letting a model guess. The judgement of which nearby pixels are the right ones is the part software is worst at and you are best at.