The level at which the swimming pool skimmers operate best is between one third and one half the way up the opening of the pool skimmer.
If the level is higher, 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 and possibly result in burning up your swim pool filter pump motor.
Add water before backwashing and vacuuming the pool because this will also lower the water level.