The Aperture plug-in BorderFX performs admirably and is often recommended. You can add text, borders, and watermarks with it, both within Aperture (as with all plug-ins, a new Master is created) or as an export.
Keep in mind that Aperture is a digital photograph developer and organizer. What separates it from Graphics programs is compositing. Everything you do in Aperture is an adjustment to the existing pixel values -- it is not designed to allow you to add data to your photographs (such as text, shapes and geometric primitives, borders, etc.). Those tasks are handled by graphics programs such as Pixelmator, Gimp, Photoshop, &c. Most users set one of these as Aperture's external editor.
