You have to consider how much it weighs, compared to how the weight is spread. Most waterbeds are about 6'x6' 36 sq. ft. So even if it weighs a ton, how much does 3 guys sitting on a couch weigh?? Say the guys weigh 200 each, the couch weighs 100, there you have 700 pounds spread out over only 12-14 sq ft.
Bathtubs are pretty heavy as well(and get filled up with that quite dense, heavy per unit of volume, substance known as water), and I always see them on 2nd floor and up apartments.