Q: How can I add watermarks to my pics in Photos?

Nope - that is not a feature of Photos - hopefully someone will provide an extension to do that but so far I have not seen it

tell Apple your needs - https://www.apple.com/feedback/photos.html

for not all you can do it export the image and use there software like Preview to add the copyright and then import the new phots

Or you can continue to use Aperture until this is provided in photos 

LN