Ideally you should keep your swimming pool water level between one third and one half the way up the opening of the swim pool skimmer. This is the level at which the swimming pool skimmers operate most efficiently.
If the level is higher than half way up, the water moving into the skimmer is going so slow that debris may pass by the opening without being pulled in. If the pool water is so high that it covers the skimmer opening, floating debris can't get in.
If the water is too low the skimmer can bottom out, thereby sucking air into the system which can result in losing the prime (water moving through the system) and possibly result in burning up your swim pool filter pump motor.