Looking at this as a 'black box', You have the heat resource of the sun that will heat all the water. Let us say 10% evaporates and condenses into the storage area, the remainder is now 3.5% salt plus .35% saltier and it maintains it's heat. If you allow 10% to exit and be replaced with cold 3.5% salt water, AND it exits via a counter current heat exchanger = heat stored is kept, the added .35% salt is a minor effluent and will disperse. The greatest problem is the propagation of algae and suspended sea life that will soon proliferate to clog all surfaces, with free floaters exiting as well. Any salt/fresh water aquarist will know that this will be a problem. The system will work until the algae problem(not to speak of clam/mussel/barnacle etc larvae that are ubiquitous in sea water.