In Aperture:

  • Save your watermark as a PSD (Photoshop) file with a transparent background.
  • Create multiple sizes of your watermark so that you can select the one that is the best match for the output size of the exported image.
  • After you create a watermark, save it as part of an export preset so you can reuse it as often as you need.

In Adobe Lightroom 3:

  • From the Lightroom menu select “Edit Watermarks …”.
  • Enter text. Hit Save.

So to use watermarks in Aperture 3, I need to use Photoshop — an expensive Adobe product?