That's not a watermark. It looks like it's just the menu option you select to take a full-screen snip not fading away completely before the screenshot is taken and ending up overlaid on it.
From some quick testing, it only seems to happen on the first full-screen snip you take after opening Snipping Tool. At least I was unable to reproduce it on subsequent snips. So try to take the screenshot again without closing Snipping Tool and see if that helps.
If that doesn't work, you can simply press the Prnt Scrn key on your keyboard to take a full-screen screenshot. Then just paste it into Paint or some other image editor and save in whatever format you want/need.
An alternative workaround is to disable menu fade-out:
- Press Win+Pause to open System properties
- Click on "Advanced system settings" in the left-hand pane
- In the Performance section, click on "Settings..."
- Uncheck the "Fade out menu items after clicking" option
- Click OK twice
In my testing, this consistently prevented the "watermark" from appearing, further confirming that it's just a lingering menu item.