Lets talk about the water pump problem where you must pump water up and out of a tank and figure out how much work was needed to accomplish this.
So assume we have a cubic container with length 1 on all sides just to make the math easy. Why is it that we assume that the force is equal to the weight of the water?
Gravity is acting downward on the water so the weight of the water should be irrelevant. Unless we are talking about how much work the earth is producing on the water which would be negative. If we are talking about how much work the pump is doing we need to know how much pressure the pump is producing on the water in the tank. Unless it is assumed that the pump has area equal to the top of the tank and is producing a force upward twice the weight of the water we shouldn't be able to answer the question.