How this works
There is no reliable way to detect an arbitrary watermark automatically, so you mark it once by drawing a box over it. We then paint over that same box - either by sampling the colour around it and blending a patch in (best for a logo or text stamp on a plain or lightly textured background), or by heavily blurring the area (safer for a busy photo, though the outline may still be faintly visible).
This is a practical "cover it up" approach, not content-aware inpainting - there is no way to reconstruct exactly what was behind the mark, only to make it unobtrusive.