There are several solutions provided by YouTube to protect content that is legally yours.
First solution, that is available to everyone, even to people who don't have YouTube channels is filling a DMCA notice to the video you believe infringes your copyright. YouTube will review the notice and eventually remove the video, plus issue a strike to the account who stole your content. This solution really helped me a lot, even if it is slow and clumsy - manually finding re-uploads, filling notices etc..
Second solution that is only available to big content owners is ContentID where re-uploads are detected automatically and the copyright holder or manager can decide what to do in a case of stolen content.
Of course there are other solutions, like directly contacting the re-uploader or his manager and resolving the issue between each other.
With my experience, watermarks won't stop people from re-uploading your videos. Some of my videos are re-uploaded as a whole, including everything from beginning to end, and some people even do compilations of several videos at once.