If your neighbor's pool water is flooding your yard and causing damage you can bring a lawsuit for damages. Your case would fall under the category of tort law.
Yes, almost all cities or counties or states have laws about this requiring pools to be secured by fences. The dimensions and other requirements differ, so consult your local building code.
You have to have their permission to connect to their fence. Usually, fences are constructed inside of the lot line, so there is some importance to what they choose to allow. In the best case...
Make up a water level, this consists of a clear plastic tube full of water with one end in a bucket of water Make sure the tube is long enough to cover the whole area of the proposed pool. you will...