Durability and dynamic failure properties are critical parameters for naval composite ships in seawater. However, previous measurements have significantly underestimated the intrinsic durability of composite structures. In this study, a simple composite seawater tank approach is developed to accurately simulate the material/mechanics conditions of composite structures in seawater. Only one surface of each composite specimen is exposed to seawater and this exposed surface is subjected to drop-weight impact testing followed by characterization of residual compressive strength. A series of experiments on dry and wet composite specimens were conducted to characterize the impact damage, and the residual compressive strength. Results from seawater exposure indicate that the compression-after-impact strengths of the wet specimens reduced by around 10% compared to the baseline dry specimens over 29-month seawater exposure. Therefore, durability of current marine composite is better than the previously measured properties. Our simple seawater tank durability experiment is expected to provide accurate justification for the required maintenance period of new composite ship structures. © 2011 Elsevier Ltd. All rights reserved.