floating, on a body of water, an apparatus comprising a first section defining a buoyancy chamber at least partially immersed at a waterline of the body of water in use and a second section that extends downward from the first section beneath the waterline of the body of water, the second section housing a plurality of reverse osmosis (RO) membrane tubes oriented vertically wherein the plurality of reverse osmosis membrane tubes are arranged around a passage and wherein the plurality of reverse osmosis membrane tubes are arranged to form an inner concentric ring and an outer concentric ring around the passage, wherein a diameter of the first section is greater than a diameter of the second section, the greater diameter of the first section as compared to the second section causing a center of buoyancy of the apparatus to be higher than a center of mass of the apparatus;

intaking a volume of the body of water, under hydrostatic pressure, into the plurality of RO membrane tubes;

generating fresh water from the plurality of RO membrane tubes; and

venting the fresh water to atmospheric pressure;

stopping, at one of the plurality of RO membrane tubes, the step of intaking the volume of the body of water while the second section is beneath the waterline of the body of water; and removing the stopped one of the plurality of RO membrane tubes from the apparatus with moving means capable of moving at least one reverse osmosis membrane tube into the passage while the second section is beneath the waterline of the body of water;

replacing the removed RO membrane tube with a new RO membrane tube with the moving means while the second section is beneath the waterline of the body of water; and

intaking another volume of the body of water, under hydrostatic pressure, into the new RO membrane tube.