Is there a practical watermark that is virtually indelible?
Covers all or most of the frame or pixels (ie., +50%)?
That would take a long time or a lot of effort to remove?  ie., repainting/re-rendering.
Doesn't trash the image terribly?
So basically its proof you have the image, but its not usable in that form.
I'm thinking - non-linear or asymmetrical coding, each image has a onetime pattern, fractal - survives scaling, noise reduction.

[note: the watermark I'm looking for should be very obvious, perceptible, overt, with clear branding or a copyright notice, ie., not imperceptible or covert]

That's digimarc:

https://www.digimarc.com/application/photography

However it is visible as a slight noise on the image and needs particular software to be read.

Maybe vivid light some grain over it through a logo mask

pixel level text outlines perhaps

or something like the dot coding that high end color copiers and printers use to imprint the output device serial number


Printer steganography
https://en.wikipedia.org/wiki/Printer_steganography