This a simplification, but any time you log onto the internet you are fair game unless you have some really good virus and firewall protection. I even covered the camera lens on my new computer with paper!! Nothing stops anyone if they are technie enough. Just make sure that no personal info gets put anywhere by you. What others do you can't control. IE: I do not put my DOB anywhere. I do use a fake one if asked by a website I really want to go to. I do not use any site that requires me to put in my SS#. I can't control what others do (insurance sites, for instance) but I can control what I put out there.
Water marking your photos is a good idea. Going to see how to do that!