Wastewater is treated on at least one apparatus positioned at a location on the surface of a body of seawater wherein a wastewater intake system takes in wastewater into the apparatus, the wastewater is treated on the apparatus to yield treated wastewater and sludge, and the treated wastewater and/or sludge is diluted with seawater prior to being discharged into the body of seawater. The wastewater treatment systems and methods can also be combined with desalination and/or power generation. In combination with desalination, the concentrate produced from the desalination process is mixed with treated wastewater or sludge prior to discharge to bring the resulting mixture to a salt concentration closer to the body of seawater than the unmixed concentrate, treated wastewater, or sludge. Combustible gases produced by the wastewater treatment process can be used to fuel a power generation system that can be used to produce electrical power for use by systems on the apparatus or transferred off the apparatus for other uses.