A method and system for separating and treating water produced from a subsea well includes separating the water subsea, and then separating the water from residual hydrocarbons on a surface vessel. The water treated at the surface, can be dumped to sea or injected into other subsea wells. The residual hydrocarbons separated on the vessel can be conveyed subsea for transportation to a processing facility along with hydrocarbons from the subsea separator. Also, the residual hydrocarbons from the surface separator can be used to fuel gas powered equipment in order to drive other equipment or to generate electricity for the vessel.