Reliability and cost-effectiveness represent major challenges for the ongoing success of composites used in maritime applications. The development of large, load-bearing, and cyclically loaded structures, like rotor blades for wind or tidal energy turbines, requires consideration of environmental conditions in operation. In fact, the impact of moisture on composites cannot be neglected. As a result of difficult testing conditions, the knowledge concerning the influence of moisture on the fatigue life is limited. In this study, the impact of salt water on the fatigue behaviour of a glass fibre reinforced polymer (GFRP) has been investigated experimentally. To overcome the problem of invalid failure during fatigue testing, an improved specimen geometry has been developed. The results show a significant decrease in fatigue life for saturated GFRP specimens. In contrast, a water absorption of 50% of the maximum content showed no impact. This is especially remarkable because static material properties immediately decrease with the onset of moisture absorption. To identify the water absorption induced damage progress, light and scanning electron microscopy was used. As a result, the formation of debondings and cracks in the fibre–matrix interphase was detected in long-term conditioned specimens, although no mechanical loading was applied.