Oral promises regarding water, power, underground obstacles, final cleanup, etc. can rarely be enforced. Sub-contractors hired by the pool builder may place a lien on your property if they aren't paid. Have the builder provide you with a performance bond. This is a signed statement that the work will be completed to your satisfaction, including payment to sub-contractors. Go to your insurance agent and buy insurance protecting you in case the sub contractors look to you for money.
Most agents offer such insurance.
Demand to see a list of pools your contractor has built. Then visit several of them and talk with the owners. Be sure the pool company has a place of business, then visit it. This will tell you a great deal you need to know. Make sure you get a written guarantee, and that the contractor will still be in business and financially able to back it up.