You want to keep your swimming pool water at a level that will allow your swimming pool skimmer to work most effectively.  This is normally 1/3 to ½ of the way up the opening of the skimmer.

Any floating debris will not able to enter the skimmer if the water level is high enough to cover the skimmer opening.  If the water level is only above the half way mark, it may not pull in the debris since the water entering the skimmer is going at too slow of a speed.

The danger of having the water at too low of a level is that it could suck air into the system thus losing the prime, if it was to bottom out.  This then would cause the failure of the pool pump motor due to overheating.

Evaporation is another important thing you’ve got to keep track of if you want your pool to be perfect for that “perfect dip”. Yes, it might not seem too obvious, but evaporation leads to a loss of thousands of gallons of water from your pool every year. That not only brings down the water level, but also disrupts the water chemistry you worked so hard on. Make sure you know your evaporation rates and compensate for that loss. Do not add a lot of water at once…add a little every day if possible. A weekly regimen also works well.