Comment by pornel

It the image is watermarked, you can't remove it that way. Watermarks easily survive uniform noise higher than humans can tolerate. Watermark data is typically stored redundantly in multiple locations and channels, so uniform noise mostly averages itself out, and cropping won't do much. Watermarks often add signal in a different color model than RGB and in a heavily transformed domain of the image, so you're not adding noise along the "axis" of watermark's signal.

For similarity search, it also won't do much. Algorithms for this look for dozens of "landmarks", and then search for images that share a high percentage of them. The landmarks traditionally were high-contrast geometric features like corners, which wouldn't be affected by noise. Nowadays, landmarks can be whatever a neural network learns to pick when trained against typical deformations like compression and noise.