What kind of watermarks? Translucent, opaque, at the bottom/top of the picture, over the stuff we'd wanna see? For pictures, if it can't be cropped out, use the heal tool in Photoshop. For videos I'd just put a box blur over that portion with Sony Vegas or Adobe Premiere/After Effects. Looks like ffmpeg has a good delogo filter too.
It's quite a bit of work, which is why they do it.