UPDATE: The instructions below on how to add a watermark in Aperture 3 without Photoshop were done using a version of Preview (the one that came with Snow Leopard). Since the launch of OS X Lion, Apple apps, including Preview, don't have a 'Save As' command. This has confused a few readers so here's a quick update.
Follow the steps below, but when you get to number 3, instead of choosing 'Save As', use the 'Export' command to export the file as a PNG. Then open that one in Preview and the Instant Alpha should be available.
He then followed up by asking his Twitter followers if anyone knew a way to create a PNG file with transparency using free software. It's a great question, especially because Aperture needs the watermark to be a transparent PNG file in order to work, but doesn't have the ability to create it.
Most documentation suggests you create the PNG file in Photoshop. Sure, a great solution. If you happen to have Photoshop.
The problem is that not everyone has it, and it's not really a viable solution for this problem. Spending several hundreds of dollars on Photoshop just to create your watermark file is not going to make people happy. I replied to his tweet suggesting you could use Preview to do a PNG with transparency by using a sort of obscure feature called Instant Alpha. The problem is creating the actual file in the first place, since Preview won't let you create one.
So, I decided to give it a try. I did find a way to use the watermark feature in Aperture without Photoshop, and using only "free" software. However, it does have its drawbacks, which I'll explain later. The method I found uses TextEdit, Preview and Aperture and goes like this:
Create a new document in TextEdit and write what you want your watermark to be. You can use any font and text symbols (like the copyright symbol ©)
From TextEdit, save as PDF.
Open the PDF in Preview and immediately Save As... and choose PNG.
Once the file is a PNG, the selection tool in Preview will have a dropdown triangle. Click and hold and a new sub-menu appears. Choose Instant Alpha.
With the Instant Alpha tool, select the background and hit delete. Do this for all background bits. Note: to select with this tool you need to click and drag. Clicking only won't work.
Once you've deleted the background, just Save. That'll create a PNG file with transparency.
Open Aperture and use this file as your watermark.
It's a bit confusing if you're not familiar with the tools, so I created the video below in which I go through the steps.
OK, so that worked. But now to the drawbacks.
The first one is that you can't use custom symbols for your watermark as TextEdit is only for, well, text. So no cool logos or anything.
The second one, and one that Joseph mentioned in a reply tweet, is that the Instant Alpha tool doesn't do a great job at selecting the background, so the edges are not perfectly clean. If you use a relatively low opacity it's not really noticeable, but the watermark may look pretty bad in certain circumstances. It's worth giving it a go though, as it uses software you already have.
Another way to create watermarks in Aperture is to bypass the in-built feature altogether and use a plugin called BorderFX (free but worth a donation). UPDATE: BorderFX doesn’t exist anymore.
Anyway, Joseph has already posted Part 1 of his Demystifying Watermarks in Aperture series and it's definitely worth a read. He's doing really comprehensive tests to figure this out and mentions he'll show us a way to do Aperture watermarks without Photoshop (or any paid software) in part 2. I'm sure he'll have a much more elegant solution that mine.