Excess moisture in a plywood or wood substrate is a probably just as damaging to a floor installation as high moisture content in a concrete slab. According to industry standards, the moisture content of a plywood substrate should be in the range of 10% to 15%. Dryer than that risks extreme adhesive absorption and subsequently poor adhesive transfer to the flooring. Wetter than that (such as through exposure to rain before installation) and the adhesives will take longer to dry; then when you have installed the flooring and the plywood has dried out, you will see the result of the plywood panel shrinkage through the surface of the flooring. A critical success factor for any floor installation is to make sure that the substrate you are working on is stable and acclimated to the working environment. Anytime you step outside that playing field, you will find yourself in trouble.