This research analyses the effect of hydrothermal aging on the water uptake and mechanical performance of biocomposites based on a polyhydroxyalkanoate matrix and flax fibre reinforcement in the form of nonwoven (NW) fabrics. The effectiveness of various surface treatments—wet/dry cycling (C), argon plasma (Ar), ethylene plasma and combinations—of these NW in the improvement of the mechanical properties of the composites is also evaluated. The water uptake during aging is analysed at both room temperature and 65 °C. Moreover, the composites are characterised before and after the aging to determine its effects on the morphology, thermal behaviour and tensile properties. It is found that the water diffusion is mainly influenced by the fibre content, and no significant differences are found in the effects of the NW treatments. Although the highest tensile stiffness and strength was found for the composites prepared with the Ar-treated NW, the C treatment is the most effective to prevent the loss of tensile performance after aging.