Mostly water marks are created for purpose, to prevent miss use of the video (to prevent piracy, to show the copyright ownership of the content, etc), removing the same is illegal.
If its not a copy righted material, there are some tools which can remove the watermark. What they do is just clone the pixels in the juxtaposition, video wont look perfect in that area (some tools just make the area pixaleted and thereby make the water mark unreadable). Just search in the net, you will find many of these tools.
Here is a similar request thread from tomshardware
If the watermark isn't that large, it may have a sence just to cut off the part of the video that contains it using any video cropping tool (e.g. VidCrop).
This may sound strange, but in some cases it really works hyeah:
I think Mohitpreet is trying to upload content downloaded from a private tracker onto another competing site or some site which has rules which prevent sharing of videos with watermarks of some other site.