What OS do you work on?
Try to replicate the issue with a sample PDF and see if that helps.
Is your Acrobat updated to the latest patch,if no ,please try to do that as well.
If there is a error message,kindly share the screen shot of the same.
The watermark feature works for one pdf after a reinstall or repair then goes back to crashing. Any suggestions would be appreciated!
I'm guessing you search for watermark in the search menu? It crashes when doing this but works (at least in my and other's cases) if you instead press the "Edit pdf" button to the right and then pick "Watermark" from to top menu.