Watermarking or Digimarking is for shit. The only advantage I see is that you can look out for your watermark later down the road and frown that someone stole your shit.
Those who steal content normally don't give a rats ass about a watermark. I know a big dutch company used to watermark their extreme content all over, and yet I saw whole webpages consisting solely of their stolen images online. Pic thieves don't get caught like this. If anyone had ever caught a pic thief with a watermark, Digimarc Corp. would tell.
Watermarks are not safe either, they can be removed the same way you remove film grain. It makes the image a tad more grainy but those who steal, and therefore get them for free, don't care.
There are even Photoshop ad ons out there made to remove the shit (
http://www.visinf.com/).