Innovation in sailing yacht design must include current environmental concerns such as resource depletion and waste management. Indeed most of the materials used today are petro-based and cannot offer viable end of life treatment. Fully biodegradable natural fibre reinforced biopolymers are being increasingly studied as they offer high specific stiffness and low environmental footprint. However, their ageing mechanisms are still not well understood. The present article gives information on 2 years natural seawater aging effect on injection-molded Flax/PLA biocomposite. Biocomposites suffer from relatively high moisture absorption which is controlled by the vegetal fibres. Simple rules of mixtures allow the determination of weight gain for flax fibres which is around 12%. Bundles of fibres and especially middle lamellae influence water uptake. Water alters flax fibres and their biocomposites, since their mechanical properties (Young’s modulus and tensile strength) are reduced with aging. A linear relationship is observed between water uptake and loss of mechanical properties. Load–unload cycles highlight damage occuring earlier than unaged biocomposites. This damage can be induced by fibre degradation and washing out of soluble components especially the fibre bundles cement, and by debonding of fibre bundles linked to their swelling.