I have started putting together different videos on photography related topics. Some of them, like the one on lighting basics, will be specifically about photography techniques. Others, like this one, will be about tools used in post production to work on or with the photographs. In these I will use Aperture and Photoshop Elements primarily. These are the two programs I use the most, with Aperture being my main tool. I only go into Photoshop Elements when I want to go all "photoshop ninja" on a photo. Most of the time I can complete all the work on a photo to finish it off right in Aperture.
There are two types of watermarks. One is the mark that goes over most of the photo. It is often like outline type or something that is lower opacity and obscures a good portion of the photo somewhat. The one that I am showing in the video is more of a trademark. It will still help to identify this photo as yours, but is not as intrusive. It is also a good way to advertise your services. It also adds that professional studio flair to the photo. How big or small you make it is up to you. But it is a really good idea to have something at least in one of the corners of your photo, especially if you are posting it on the web.
In the Aperture watermark video I do talk about the scale watermark feature. This feature took a bit to figure out. It is quite nice in one respect. You can create a watermark that you can then use on exports of both full size images and scaled down images and the watermark will look the same in both. But you need to size the watermark to your expected full size export. You might want a few copies that are a little smaller for those photos that you cropped and are smaller than the original size out of your camera. If you have more than one camera and they have different original photo sizes then you will want watermarks sized to those photo sizes. So if you have a 10 megapixel camera and a 16 megapixel camera, the original photos will be different sizes, requiring different size watermarks. It would be nice if Apple would create one other option to scale the watermark in a particular section so that it would fill the particular section, scaling the photo to the section and not as a ratio to the full photo. Did that make sense? It did in my head.