This paper describes an approach allowing aging to be included in the design of marine structures. Two types of aging are considered, physical aging and seawater saturation. The latter is shown to be more severe for this material, with losses in properties of up to 30% being measured after aging. A complete set of in-plane and out-of-plane properties was generated for a carbon/epoxy composite, and these were then used as input data to model the response of a flexural beam under a mixed shear/bending load. Reasonable agreement is found between model and test results; both show the strong influence of seawater conditioning on the composite performance, with a reduction in strength after aging of around 35%. This approach can be applied to account for seawater aging during the design of composite structures.