You've hired the contractor and he has done the excavation and installed your pool structure. The plumbing and electricity is hooked up and the floats are blown up and ready to go. What's left? The water, of course. Filling a pool may seem like quite an easy process. Simply grab the hose, turn on the water and go. However, a swimming pool requires a whole lot of water, and a hose may not always be the best solution. Even if it is, you need to factor in the cost of using that much water to fill up your pool.