I have a full flotation waterbed. And when one person moves, everyone moves. Now I don't know if that dad's report and my experience have any scientific value but if you think about all the motion your body experiences in a full flotation waterbed, maybe there's something to it.
I have heard some people say the full flotation waterbed gets them seasick. Hmmmmm.... But what I do know is all the waterbed stores that used to be around here have long vanished. Maybe there wasn't enough of a profit margin as most of us hippie era kids built our own frames and just bought the $50 mattress.