We're really not supposed to remove watermarks, are we? lol but I know when I use an image that has something in it I don't want (like... another person in the background, or smudges on the person's self or watermarks) I usually use the Push or Smudge tool. Push when I use PSP, and Smudge on PS.

Just... zoom in as much as you need, and drag the closest colour over the watermark. Set the brush to a small size, like 2-3 depending on the amount of detail in the picture. After you've done this (you'll need to be very patient), just use the Soften brush, set with a low opacity and go over it to blend it in.

There's probably a better way to do it, but this is how I've always done it. Maybe someone else has another suggestion?