Or... buy some clear plastic tubing from a Home Store and fill it with colored water (food coloring added to tap water). Make certain there are no air bubbles in the tube.
Drive a stake on the high side of your pool and a second stake on the low side of your pool. Attach one end of the tubing to the high stake (with wire or tape) and attach the other end of the tube to the low stake. Then grade the earth until the height of the water level in the tubing from the earth is the same on both stakes and all around the pool area.
As you found out by filling your pool, water levels don't lie.
The string level should work fine for your application, but the water level is another way to go, and it is very accurate over long distances.