Desalination plants esentially push water through a membrane at high preassure. The salt(with a lot of watter) stays in the filter(is trown away) and fresh water cones from the other side.

A preassure diferential is esentialy the (heigth 1x density 1 - heigth2x densuty2 )x a constant.

Since fresh watter is ligther than salt water if we build a straw long enough then the preassure diferential will be enough to push the heavier salt water into the fresh water filled straw.

I did the math a while ago, the straw has to be about 75 km long. For the moment lets ignore that at those depths we are probably not dealing with water any more so the membrane does not work. Lets supose it does.

This contraption still violates the laws of termodynamics. So where is the mistake? My best guess is that the filtering action should push the straw down until it is at sea level. And you need to constantly apply work to keep it in place but i am not sure.

In a fantasy setting you could pribably apply that work by using tidees or bouyant forces or something.