You can visit the answer of this question: https://www.researchgate.net/post/Where_do_I_get_image_watermarking_standards_from

Visit GitHub for image watermarking data.

Some example link:

https://github.com/marcbelmont/cnn-watermark-removal/tree/master/assets

https://github.com/rohitrango/automatic-watermark-detection/tree/master/coco_dataset

https://github.com/rohitrango/automatic-watermark-detection

https://github.com/marcbelmont/cnn-watermark-removal

Another source.

http://www.petitcolas.net/watermarking/image_database/index.html