If our Lumbar Supreme waveless waterbed mattress is properly filled at approximately 8 1/2" it is a firm and Lifting waterbed mattress because of the special lumbar support layers. The Boyd Lumbar Supreme 98% is so buoyant you can't sink down. Patented hi-loft fiber makes it very firm yet pliable enough to confirm to your shape. Entry and exit is easy. Motion typically stops in less than one second. Sleepers motions do not effect each other at all. 3 Lumbar support fiber layers to support sleepers midsection for a very flat sleeping posture.