Water can stratify if the layers have different temperature, salinity, or change in density from other causes. https://en.wikipedia.org/wiki/Halocline https://

Water can stratify if the layers have different temperature, salinity, or change in density from other causes. https://en.wikipedia.org/wiki/Halocline https://en.wikipedia.org/wiki/Halocline If there is no convection or wave-action to mix the saltwater with the fresh, they can coexist in the same vessel, with very slow diffusion of salt into the fresh layer. If the fresh layer is refreshed by rainfall or fresh outflows of groundwater, the top may always be drinkable, even as the bottom is not.