Use caution and never drain a pool before having a solid plan. Depending on the manufacturer of the fiberglass pool as soon as you drain it you likely void any warranty it may have had. A fiberglass pool can cave in, so bracing with 2×4’s is critical. Also be prepared as you are for ground water issues (sometimes holes need to be drilled in the bottom of gunite pools to relieve hydrostatic pressure). You could encounter something similar with a fiberglass pool. Only drain when the weather conditions are dry – and there is no standing water in the yard.