I don't know about actually adding water... But they definitely try not to lose any. When I did industrial refrigeration classes they stressed the importance of keeping humidity up in meat cutting rooms. That's why beef jerky is so friggin expensive!
Ya, I don't think they add water that I have ever heard. The better quality ground beef (and lower fat content) is usually less moist. (que Karl)
I don't like 90%. No fat, no fun and no flavor. The 70-80% include a lot of fat trimmings. Ground round or ground sirloin not so much. Some of that fat stays inside the burger adding moisture and flavor. McDonalds used beef tallow for years to fry their french fries in. That's why they were so good back then. If you buy the cheap stuff you may get a few extras. Look up "pink slime".
