Well, Aperture can add a watermark to your exported photos for you, however, you have to create the image in Photoshop or another program first!
In the browser bar, right click the image you want to export. ; Choose "export". ; Choose a format, but edit it. ; From the settings screen for the export option you can select/deselect adding a watermark.
Personally I use BorderFX which is a free add-on for Aperture, which can add EXIF data to the photo during an export. ; I use it to add copyright year/name. ; Which I can batch add using the metadata stamp tool.