I usually watermark my images for a number of reasons. The two main reasons are:
1) Not really relative to this thread but as a Professional Sports Photographer, not my primary job, I am under strict legal restrictions associated with the credentials provided me in order to shoot professional events including the NFL, MLS, MLB, PGA, NCAA, International Soccer, etc. To protect myself I watermark my images because if my images are used out side of the T's & C's my credentials will be pulled and future access denied. On my portfolio web hosting site or other public sources I showcase my images I add a disclaimer similar to;
The images are from the portfolio of GRH. They are presented here for your viewing pleasure. These images may not be copied, linked to, printed or used elsewhere in any way and are protected by copyright and the terms and conditions of the NFL.
2) While I make some money as a Professional Photographer, I shoot out of a passion for Photography and have an income without it. I work with and know a lot of people where Photography is their main source of income. Not much of an issue in highly regulated T’s & C’s associated with Professional sports where I'm protecting myself, but in plenty of other areas out of respect for those photographers where photography is their main source of income, I don’t tend to give my images away so as not to take business away from them and I watermark them so they aren’t used without consent or at least someone needs to make an effort to clone out my watermark..
I don’t enter contests for a related reason, just read the contests T’s & C’s and if they take full ownership and rights to use and sell your images without compensating you, I am not interested.
With so many people having access to quality equipment, and many of these hobbyists not interested in making money but thrilled to get published or recognized, business for professionals is getting harder and harder.
If you visit photography forums catering to professionals you will read very spirited discussions unhappy with amateurs giving away their work Even when the quality is not up to the level of professionals it is difficult to compete with free. Add the reality that many amateurs produce professional level work and you can hopefully understand my sympathies.
I’m not judging those that choose to give their work away and don’t think badly of them if they do, I just personally choose not to in most cases for the reasons above.
And as a note, all your images are protected by copyright but in order to sue for copyright protection your images need to be registered.