A flexible structure assembled of impermeable modules surrounds a constant volume of liquid so as to control its quality. The modules may join together so that they can adjust to changing water levels. They may have a single layer or an impermeable membrane may sandwich a foamed core. Anchor sacks may hold the structure tightly to the sea bottom, or conventional anchors embedded in the sea bottom may anchor floating structures whose modules close their bottoms. Seals over joints reduce seepage so that conventional filters may clean and treat the liquid. The foamed core modules buoy the structures and insulate against heat transfer. A floating mat of modules comprising flexible transparent membranes may cover the enclosed liquid to prevent heat loss while achieving a solar energy gain. The structure may also be made in one piece from a flexible membrane. The structures are useful for safer bathing, aquaculture, recreational diving, underwater work or exploration, filtering operations and isolating liquids for any other purpose more easily, quickly, cheaply and safely than conventional structures.