I understand that there is a business cost involved with supplying the imagery, and a cost in developing the technology, storage of the data, etc. and that you want credit for that. I'm in the same exact situation with this site. There is a lot of public domain material on this site. Some of it I will watermark, but not with any sort of mark that implies that I own copyright on it.
Do you differentiate between imagery that is in the Public Domain and not in the public domain when you generate the watermark? The data that is not in the public domain, did you purchase the copyright for it?
I don't mind the gray watermark with the name of the site on it. That's perfectly reasonable. But watermarking the image with a (c) NETR, LLC implies that the image itself is copyrighted by you, when there are certain instances that it is not.
I don't want to disparage what you're doing -- it's a terrific service and will be invaluable to people interested in history. The double watermark, and the incorrectly copyrighted public domain material just doesn't look good.