In this paper the optimization of the main design parameters is performed for a double-stage seawater reverse osmosis (SWRO) system coupled with an ultrafiltration process; the minimal energy consumption is the goal of the optimization process. The energetic benefits for the membrane process are calculated, deriving from the use of a hybrid two-stage/two-pass lay-out with ultrafiltration pre-treatment, which is a typically a low energy consumption process. Also the benefits on membranes' efficiency and duration are assessed by examining the very low Silt Density Index (SDI) of the water exiting the ultrafiltration section. The analysed system includes a SWRO membrane in the high pressure stage and a BWRO membrane in the low pressure one, according to a well-known practice for small and medium size plants. Such plant lay-outs allow to threat high TDS feedwater (up to 40 g/l), usually adopting very cheap membranes and operating them with a very low recovery factor. A comparison with a single stage RO section is performed, assuming a typical TDS of the seawater for Mediterranean regions and a fixed TDS level for the final product (< 500 mg/l). The investigation mainly concerns technical and energetic aspects, but a brief cost analysis is finally presented in order to assess the economic viability of the proposed solution.