Abstract: People-supporting base assembly with fluid element, in particular water, comprising at least a fluid-filled mattress capable of being filled with liquid medium through a filler fitting and being adapted onto support frameworks of beds and/or settees, or onto the floor, and an enhanced-flexibility mattress possibly laid upon said fluid-filled mattress, and also comprising at least another possible support structure (8, 22, 36) that is adapted to be placed upon said fluid-filled mattress (6, 48, 53), and to be covered by the possibly provided enhanced-flexibility mattress, said further support structure (8, 22, 36) being provided with elastic means (9, 19, 40) adapted to sustain people in a springy and anatomically correct manner by adapting the surface conformation of said structure in accordance with the actual weight of and the posture taken from time to time by the person(s) lying thereupon. Various embodiments of both the mattress (6, 48, 53) and the support structure (8, 22, 36) are described.