Instead of using Photoshop or other complex tools to make watermarks I use TextEdit and some simple techniques. I don't get text with transparency this way: everything created will have a background of some sort, but it is quick and effective (and free).

First I launch TextEdit and type in the text I want, centered if that is the effect I am after:

Then I select all the text and press command T to bring up the fonts panel:

and play around until I get what I want:

That is Snell Roundhand 36 pt. Very nice. I can also use the character palette to find other special characters and decorations. To get the character palette, turn it on in the International Preference Pane:

and click the checkbox at the bottom that displays it in the menu bar:

And then select it from the menu bar. I pick the character I want:

and press Insert to get the character into my document:

Now it looks the way I want it, I need to save it and take a snapshot of it by pressing command shift 4 and dragging a rectangle around the text like this:

That gives me a file called Picture 1.png on my desktop. That is the snapshot.

To set up this image as a watermark, I select an image and go File > Export > Export Versions and select Edit... from the Export Preset pop-up menu:

Then I create a new preset with the + button:

or select an existing preset. Then I drag the image file from the desktop into the watermark drop box:

To turn it on, I check the Show Watermark checkbox, I pick the location, and set the opacity to get the right look:

Scaling the watermark will apply it before the image is resized for export, resulting in a small watermark if a large scaling is used. Not scaling will keep the watermark the full size.

Clicking OK to close and Export to do the export I get my watermarked image:

For a different effect this can be inverted or have different colors. I go back to the TextEdit document, select the text and go File > Text > Table... to create a table that includes the text in a cell. Changing the table Rows and Columns counts to 1 and 1, gets me this:

It does not look very different, but since the text is now inside a table, I can change the text and the table cell background colors (and the cell border). I select the text and hit command shift C, then pick white from the Colors panel. Then I set the cell border to 0 px and select Color Fill for the table cell background:

Now I click on the color patch on the right and pick black from the Colors panel. The result is white on black which I can take a snapshot of as before:

And here is the final result with the transparency turned way down:

It would be a great addition to the watermark features if Aperture had a way of selecting a color range as being transparent and had the ability to invert the image it is given.