I'm not really sure what you're trying to prevent given your example. If you're showing a presentation to a client in a closed meeting, are you scared the client will pillage the design? Do you have a contract in place? If you're not doing spec work, a presentation would likely happen after you've signed some formal contract for the project and thus any theft of that design would mean litigation -- an easy case which you would win in court.
Are you hosting the presentation online and are in fear of someone coming across it and stealing it from your public link? First and foremost, design theft isn't as common as new designers might believe. Inspiration and borrowing ideas happens often, but straight out theft is actually rare; couple the rarity of that occurrence with the chance someone would happen along your work organically from something like a generated Dropbox link (also very slim), and you have a near improbable scenario that you're losing sleep over.
Again, I'm not sure what the goal is, but in 20 years working as a designer I've maybe seen a few designs poached and it never robbed me of any tangible income. First, the type of individuals that do this aren't going to last in the business and are likely charging pennies in comparison, working for equally "cheap" clients that wouldn't have made it on your client list anyways. Second, it just doesn't happen often enough to take big steps to prevent it from occurring. Third, and I apologize for being candid, unless you're the next upstart design genius, it's unlikely that anyone is really stealing from you anyway. If theft does occur, it's usually in the form of modified reproductions of popular works, from popular designers. It's not really going to happen to new talent. I'm not trying to be rude, but it's just the truth; I can tell by this and other questions on the forum you're relatively new to this business, so I wouldn't worry too much about having your work stolen and reproduced.
I think you're worrying about the wrong thing here, unless you have a very specific case where stealing is somehow more likely. Don't give yourself anxiety over something that may never happen, even for the entirety of your career. Learn to be a better designer, spend time honing skills and practicing your craft rather than fretting about how to apply watermarks to your projects.