Apparatus and methods for producing potable water from an aqueous feed such as seawater utilizing a pressure vessel designed for operation in the range of about 250 psig to about 350 psig having reverse osmosis (RO) separation elements disposed upstream of at least one nanofiltrate separation element. The supply of seawater at such pressure creates no permeate flow through the RO membranes, so the seawater flushes the feed side of the RO elements on its way to the NF element where very substantial permeate flux occurs, producing a NF permeate substantially depleted in divalent ions and optionally moderately depleted in monovalent ions. The NF permeate is collected in a holding tank having liquid level switches, and when a desired level is reached, a control system causes the pump to draw suction from the holding tank while shutting off feed from the seawater source. When the NF permeate is supplied to the feed side of the RO elements, its osmotic pressure is such that substantial RO permeation now occurs creating a stream of good quality potable water. During both stages of operation, energy is conserved by routing the concentrate stream leaving the pressure vessel through an energy recovery device that boosts the pressure of the liquid being discharged from the positive displacement pump.