•Wire. Whatever goop makes up this membrane is not going to be able to send all that energy back to the desired destination. I'd expect you to have to come with some kind of metallization and tabbing not unlike what you find on solar cells.
•Electrical insulation. At least with solar cells the plus and minus sides are pretty well insulated from each other. Wet goop on the other hand, not so much, and with water occasionally flowing around the edges to shunt the power produced.
•MPPT. I'd imagine this produces a highly variable low DC voltage, which is guaranteed to not be even close to the highly stable high AC voltage you want to provide, so you need these boppers. Luckily, PV has made them cheap. Unluckily, they're not waterproof.
•Lots of pressurized freshwater, and lots of pressurized saline water. Eventually, it'll all be saline, unless you have streams or rivers or other naturally charged sources. When it's all saline, power output goes to zero. When it's the dry season and the river dries up, power output goes to zero. When sea levels go up a foot, well, you know.
In the end, the amount of available surface area, if we spent trillions on R&D to make the perfect design of these, and resigned ourselves to killing all the species at this halocline and making river mouths nonnavigable, would probably be rated less than a couple conventional hydro plants. Efficiency is a red herring, and many devices (thermoelectric generators in particular) become more useful the less efficient they get.