I will provide all who visit this page with a working answer, having in mind that this is now 2015:
Of course you can have you images uploaded to a Google+ album and still have you originals on your computer. Supposing you have uploaded the images with Picasa 3 and you still have the connection in Picasa with both the local and online album.
What you need to do to ad a watermark, change a watermark or remove a watermark from any already uploaded images is to have Picasa overwrite the online images with a fresh copy. One way of doing that on a PC is:
Set up Picasa to ad whatever watermark you want or have to set no watermark at all.
Select all images in the local album, Press Ctrl + R (turn all of them 90 deg right), let Picasa sync everthing in the album, then press Ctrl+Shift+R (turn them back again), and then let Picasa sync everthing again.
All images in the online album will now be updated with any new watermark settings you applied before the procedure.