What video? what watermark? Do you actually have permission to use the video? You know, there are legal implications here.
In general, this is pretty difficult, if not impossible. The only way to do this is probably a lot of cloning/painting, assuming you have some clean material to actually use as a source.
As Mylenium pointed out, if you're asking about this as a way to get around not having permission to use the video... don't. Not only can this have legal repercussions.... it's SO not worth the time.