Taken from looploc.com
Should I drain my pool before putting on my LOOP-LOC cover?
No! It is very important to maintain proper water level to help support the weight of the cover and prevent excess wear. For LOOP-LOC mesh covers, water level should be 15" to 18" below the top of the pool. For ULTRA-LOC solid covers, water level should be 12" to 16" below the top of the pool. If proper water level is not maintained, the warranty can be void.