Woven and non-crimped glass fabric reinforced polyester flat composite plates are studied experimentally. Water immersion tests for undamaged and impact damaged specimens are performed. The accelerating effects of increased water temperature on the degradation rate are discussed. The effect of up to 30 months water immersion on the impact behaviour are extensively investigated and compared with pre-water immersion results. The effects of different E-glass fabric reinforcement types, woven and non-crimped, in terms of environmental and impact behaviour are reported. An Environmental Damage Accumulation Metric (EDAM) is proposed and analysed, linking the time of water immersion with a marked change of behaviour in terms of the loss of the elastic component during the impact event.