This paper proposes a new floating sub-atmospheric solar water desalination system to provide the water resource for domestic to industrial usage. In addition to conventional solar stills (CSS) which only utilize incident solar radiation, the sub-atmospheric condition of this system greatly increases the productivity of the system. The mathematical model of the system is developed and the performance of the still is evaluated. The overall productivity of the still is shown to be sensitive to evaporator pressure as the most significant parameter of the system. Other parameters affecting the performance include cover transmissivity and input water temperature.