Adobe Photoshop is the only way. They have a 'smart tool' in there where you just highlight your region, and an algorithm will do its best to fill it in. Pair it with other tricks and you can remove any watermark and have flawless image at the end. It will take some skill to get good with it.
If you have lots of images all with same watermark near a border, you can use software like ''faststone photo resizer'' to trim 20px off bottom border for example. Very handy for bulk image processing.
Last resort...pay someone on fiverr to do it.