I lived in an upstairs apartment of a 175 year old house. I also had a waterbed up there.
Sort of worried about it cause the floor started to sag about an inch in the first year. Anyway... The owners 300+ pound daughter kind of "liked" me. I invited her up to hang out, and watch a movie/dinner. She saw the waterbed, ran / jumped, and landed... My heart stopped. The floor held.
It would have landed in her apartment anyway.