I would actually use the most blatant watermarks all over my photos, since I know it would definitely suck to see my photos all over Ebay or some scam site, being used to sell shoddy products. Even blatant watermarks/logos don't prevent theft; I've seen some pretty ugly cover-ups and some where the original wasn't even covered up... But hey, do what we can, right? Even if it mars a beautiful photo, better than having it stolen.
RIP CosRevo, Love Laugh, Sweety Pop, Cocosilver.