Glass fiber-reinforced polymer composites (GFRP) were prepared and reinforced with two types of multiwalled carbon nanotubes, i.e., amino-functionalized (ACNT) and pristine nanotubes (PCNT). Incorporation of ACNT leads to a significant decrease in equilibrium water content and diffusion coefficient vis-A -vis GFRP composites when immersed in seawater (SW) and distilled water (DW). The experimental results illustrate that the material degradation of the composites is significantly more on immersion in SW as compared to DW. Interestingly, a decrease in mechanical properties was observed initially; however, at a later stage the trend reversed and a recovery in the mechanical properties was seen. This recovery in mechanical properties is ascribed to the possible secondary crosslinking network between excess sodium ions in seawater with the hydroxyl groups of cured epoxy and also to the unreacted silanols on the glass fiber surface. On the contrary, PCNT composites revealed a continuous decreasing trend in mechanical behavior versus aging duration. The present study suggests that it may be advantageous and safer to re-condition (for 15-20 days) FRP nanocomposite parts before being installed in naval vessels.