The aim of this study is to investigate the usability of carbon nanotube (CNT) reinforced plastic pipes in seawater and consequently the changes in mechanical properties. The glass / epoxy tubes used in the experiments have fiber orientation of [± 55]3 and are produced by filament winding method. 1% CNT was used as secondary reinforcing material. The test specimens were subjected to tensile and hardness tests after being kept in seawater for real periods of 2,3,4,6 and 9 months in addition to dry environment. Tests were performed on ring samples according to the split disc method based on ASTM D 2290 test standard. The results were compared with the results obtained from glass reinforced plastic (GRP). According to the results, it was found that the production of CNT reinforced composite structure reduced the delamination damage and as a result of this it provided 21% strength increase in dry environment conditions and also increased the strength in long term usage by approximately 18%.