an underwater buoy for hanging a duct between a sea bed and the surface , the buoy including a frame and a plurality of modular members mounted in the frame , the modular members extending between their two opposite modular - member ends , the frame including a longitudinal body for receiving the extended tubular duct , and retainers mounted on the body for maintaining the modular members substantially parallel to the body . the retainers include two retaining structures spaced from each other along the body . the retaining structures are maintained in a fixed position relative to each other in order to hold at least one modular member .