Composites used for marine applications are subjected to various environmental effects, such as moisture, temperature, UV radiation, and seawater. In this study, effect of seawater on the degradation of mechanical properties of conventional and nanophased carbon/epoxy composites was investigated. Epoxy resin was modified using 1 wt. %, 2 wt. %, and 3 wt. % nanoclay. Carbon/epoxy composites were fabricated by vacuum assisted resin transfer molding process and compared with neat samples with and without exposure to seawater. Nanoclay was dispersed into matrix by using magnetic stirring. Mechanical characterization performed through three point bending tests showed that 2 wt. % nanoclay loading was optimum. Flexural strength and modulus were increased by 25% and 12.51%, respectively, compared to neat system for samples not exposed to seawater. Flexure samples exposed to the seawater for 30-, 60-, and 180-day periods revealed that samples with nanoclay retained better mechanical properties compared to neat samples. After 30-day exposure to seawater, there was no significant reduction in the strength and modulus. However, flexural strength was reduced by 10.24%, 7.08%, 5.28%, and 7.13% for neat, 1 wt. %, 2 wt. %, and 3 wt. % nanoclay-infused samples, respectively, after the samples were exposed to seawater for 180-day. At the same time flexural modulus was reduced by 12.61%, 7.16%, 4.59%, and 6.11%, respectively. From scanning electron microscopy (SEM) studies, it was found that failure occurred due to delimitation and initiated from the compression side. Nanophased composites exhibited better bonding between fiber and matrix. SEM micrographs also revealed that both unconditioned and conditioned nanophased epoxy, which produce relatively rougher fracture surfaces compared to neat samples. Optical microscopy study revealed no significant physical change in outer surfaces of the samples conditioned up to a 90-day period.