Hi @mgco! Welcome to the community!
I am in a similar OS state as well (Win7 at work and OS X at home - linux on various VM’s across the two).
This is one of the reasons I primarily work in RawTherapee. I like being able to work on images during lunch breaks at work and to be able to pick up and continuing processing at home without skipping a beat. Though I have been known to dabble in darktable from time to time. 
My general workflow focuses on specific tasks that I’ll usually need to accomplish:
- Quick contact-print view of all the images.
Initial pass at exposure validating, focus, and composition.
Usually done lately in native OS file manager or using imagemagick montage to create an actual contact sheet image. - Validation + work in raw processor.
Verify focus and composition, sort by star ratings to winnow the results down to potential keepers, work on exposing the raw. - Export for further processing depending on workflow.
- GIMP + G’MIC for various types of retouching
- Hugin for panorama stitching or pixel-bending or creating lens correction parameters
- Luminance HDR (usually happens earlier in the workflow lately with HDRMerge).
- exiftool to maintain Exif data through the various pipelines
- imagemagick for any batch-processing needs
- Bash (batch renaming/moving/copying/etc…, processing needs)
In this case, why not just stay in darktable all the way through the workflow? 
Honestly, I’m not a fan of watermarks personally but if you need to add them for sure imagemagick is a super-fast and easy way to do it with a single line in your shell. Since this is usually the very last thing done to images it’s often something I’ll leave to imagemagick in batch as needed.
If I get a moment soon I can test a quick imagemagick script to apply a watermark and post it back here in case anyone wants it. (Pretty sure it’s a simple composite command, or a mogrify + composite overlay).