A high salinity feed water such as seawater is treated to produce a reverse osmosis (RO) concentrate and an RO permeate. Optionally, some or all of the RO concentrate may be filtered to produce a nanofiltration (NF) permeate. Optionally, some feed water can also be filtered to produce NF permeate without first being concentrated by RO treatment. The NF permeate, or a blend of the RO permeate and NF permeate, may be used to produce a product water for injection into an oil-bearing reservoir to enhance oil recovery. Optionally, the product water may have salinity greater than the feedwater, or at least 30 g/L. The product water may have hardness of less than 20 mg/L.