The performance of the fiber reinforced epoxy nanocomposites incorporating silylated clay minerals exposed to seawater ageing vis-à-vis the amount of grafted silanes has been studied. The novel glass fiber reinforced epoxy nanocomposites incorporating clay minerals treated with different concentrations of two different silane coupling agents (3-Aminopropyltriethoxysilane and 3-Glycidyloxypropyltrimethoxysilane) have been exposed to simulated seawater at two different temperatures i.e. 25 °C and 55 °C. All fiber reinforced epoxy-clay nanocomposites showed an increase in weight due to seawater absorption. The water uptake studies were analyzed with an assumption that weight loss due to hydrolytic degradation of epoxy would be insignificant as compared to weight gain by fiber reinforced composites due to absorption of seawater. The rate of diffusion of seawater in glass fiber reinforced epoxy-silylated clay nanocomposites was slightly less than that of glass fiber reinforced epoxy-clay nanocomposites incorporating a commercially available organically modified montmorillonite Cloisite 15A®. The water uptake behavior and durability of nanocomposites were significantly influenced by the polarity of the organic moieties, the morphology of clay minerals in epoxy and temperature of seawater. Clay minerals modified using small quantities of silane coupling agents in fiber reinforced epoxy-clay nanocomposites led to better retention of tensile and flexural strength after hygrothermal ageing owing to silanization effect on exfoliation, as well as forming covalent bonds with the matrix. However, if silane coupling agents are used in excess, it will lead to the development of siloxane network which will also encapsulate clay minerals. The presence of such networks would act as concentration sites and lead to a significant decrease in tensile and flexural strength, as observed. © 2017 Elsevier Ltd