All comments seem to focus on the cost/energy requirements of the filtration process.. but the elephant in the room is the waste. How to safely dispose off the waste water after filtration completes? That water is virtually useless
Run long tubes (1-3mi) out into then ocean with perforations so you can diffuse the highly salty water back into the ocean without disrupting local ecosystems.
Ocean-going vessels could be fitted to take on brine as ballast in port and gradually replace it with sea-water during their voyage.
This has the useful side effect that it should prevent the transfer of invasive species in ballast tanks, as most cannot survive high salinity conditions.