Most disappointing results come from a handful of avoidable choices. In rough order of how much they matter:
1. Start from the best source file
The single biggest factor. An original PNG beats a re-saved JPEG, which beats a screenshot, which beats an image saved from a messaging app. Compression damage around the mark has to be repaired along with the mark itself.
2. Select slightly beyond the mark
Watermarks have soft edges. A selection drawn on the visible boundary leaves a faint outline behind — the part of the mark too dim to see clearly but bright enough to survive. Two or three pixels of margin removes it.
3. Take the source from the same surface
Not merely a similar-looking area — the same material, under the same light, at a similar angle. A wall patch taken from a different wall brings the wrong texture with it, even when the brightness matches.
4. Work in several small patches, not one big one
A large selection asks the tool to invent a large area at once. Two or three smaller repairs, each with its own well-chosen source, almost always beat one sweeping selection.
5. Do not scale the image first
Repair at full resolution and downscale afterwards if you need to. Downscaling later hides residual imperfection; doing it first destroys the texture the repair needs and magnifies what is left.
6. Zoom out before you judge
At 400% every repair looks slightly wrong, because at 400% every photograph looks slightly wrong. Judge at the size people will actually view it. It is easy to spend twenty minutes perfecting something invisible at normal scale.
7. Save as PNG while you are still working
Compress once, at the very end. Intermediate JPEG saves accumulate damage that each subsequent edit has to fight.