Watermarking will not protect anything, it would only make possible to trace a copy to the original purchaser. But anything that can be read can be "broken" too.
If watermarking is what you want, this might give you some ideas:
https://www.mobileread.com/forums/sho...8&postcount=11
https://www.mobileread.com/forums/sho...4&postcount=13
That's mainly for ePub, where the code is stored as-is. I don't know how much of it would be applicable to mobi files.