This is the original "big wave" experience. The water moves freely, oscillating for several seconds after you move.
Waterbeds require an electric heater to keep the water at a comfortable temperature (usually 85–90°F). Without it, the water will pull heat from your body, making you dangerously cold.
The biggest draw of a waterbed is its ability to contour perfectly to your body, eliminating pressure points. However, the "feel" depends on the internal construction of the bladder:
A filled king-sized waterbed can weigh over 1,500 pounds. You must ensure your flooring can support that concentrated weight.