Create a jpg file containing the text you want. Aperture can't process text so you'll need to find another way - sensible selection of font could allow you to prepare in textedit then do a screengrab. Remember where you save this file as you'll need to call it later.
When exporting your photo (the image you want to watermark) there's an option in the dialogue box for (i think) export preset. Should offer you a list of several settings like Original Size, 50%, tif etc. Scroll down to the bottom of that list and select Edit...
You'll now have all your export settings in front of you. Towards the bottom right of the dialogue box should be the Show Watermark option. Tick that, then Choose File. This is where you call your watermark file you saved earlier.
You have options for adjusting transparency and position, and there's also Scale Watermark which reduces your mark to fit in a corner rather than running full width. It's a case of experimenting to find out what works. Pretty crude in the overall scheme of things but workable. I find white text on black background with low opacity (high transparency) works best.