This paper presents results from an experimental study of the mechanical behaviour of flax fibre reinforced acrylic composites after immersion in seawater. First, in-plane design data are presented. Then, seawater diffusion kinetics are shown, the influence of seawater on design properties is quantified and flexural fatigue performance is described. Finally the results are compared with those for glass reinforced composites. Seawater saturated specimens show a large drop in stiffness (>50%) due to fibre plasticization, but smaller drops in strength (<30%). The relevance of immersion tests is discussed and an alternative unifacial exposure test is recommended for these new materials for sustainable marine applications.