Looking at the source, it appears that the low-level method would accept a page parameter that you could pass as "1" to watermark just the first page. However, the PDFWatermarkActionExecuter class does not expose the PARAM_PAGE parameter.
This would be an easy change to make in your own fork of the project. If you do that, submit a pull request because that's a legit enhancement.
If you can't make the change and the pull request, at least create an issue requesting this as an enhancement and one of the people who helps maintain the library will get to it.