Watermarker (OS X 10.7 and later, US$7.99) offers a functional way to apply image watermarks. For small batches of images without a lot of customization demands, this app probably does what you need it to.
The features are basic: you add text, you add a logo, you save the new image. The app offers standard options like transparency control and color selection. There's also a button that inserts the © character for you.
Importantly, you can save your choices as presets, so you're ready to re-use already designed watermarks for future projects.
Anyone looking for advanced tweakage such as, for example, putting the text at the top or bottom centers may be disappointed (there are some placement options, however). Watermarker boils its task down to the bare bones. It does 90 percent of what most people need it to do and it does it in an interface that's easy to understand.