One of the major factors affecting the acceptability of novel coating in engineering applications is the degradation of the coating by the moistures that will affect the physical and mechanical performances and causing it to fail. Nanoscale reinforcement provides opportunities for enhancing the polymer system with unique properties and increases the performance. These advantages not only lead to an improvement in the mechanical properties but also act as a barrier to slow the water permeation. In this study, an organically modified montmorillonite (MMT) nanoclay and a further modified “X-treated supernanoclay” were used to reinforce an isophthalic unsaturated polyester gelcoat. Five gelcoat systems were prepared and characterised by using a boiling water absorption test (with a saturation time of 12h) to characterise and quantify the degree of water absorption. All nanocomposite samples show a very convincing result in terms of the coefficient of moisture absorption and reduction in water ingress as a function of weight fraction of nanoclays. The “X-treated supernanoclay” reinforced systems show the greatest improvement compared to conventional nanoclay system.