Because those connecting fresh water bodies are higher than the sea. So the cheap solution is to fill progressively higher boxes with water from the fresh water

Because those connecting fresh water bodies are higher than the sea. So the cheap solution is to fill progressively higher boxes with water from the fresh water, until you are at fresh water. And then do same thing in reverse. Perfectly fine when you have enough fresh water, you can even somewhat optimize by using same water multiple times. But if you run out of fresh water at high point you will have trouble.