If the watermark is at the very top or bottom, use a free tool like Handbrake or Shutter Encoder to crop those pixels out. Note that this will change your aspect ratio.
If you aren't ready to invest in a license, there are several powerful, truly free (open-source) editors that do not add watermarks: created with a non-activated version www.avs4you.com png
At first glance, this seems harmless. It is just text, right? But in the world of professional content creation, that line is a landmine: If the watermark is at the very top
: Activating one AVS4YOU program automatically activates all other installed AVS4YOU programs on your system. Re-render Your Project there are several powerful
: It acts as a digital sign to verify the software hasn't been legally activated with a license key Persistence