How would you find the mass of water in a swimming pool?

You would measure the Swimming Pools length, width and depth to determine how many cubic feet of water is in the swimming pool. Length x Width x depth = cubic feet. Then measure the mass of 1 cubic foot of water by building a box 1'x1'x1'. Once you know the mass of 1 cubic foot of water - you'll know the mass of the water in the swimming pool.