The hull of an Autonomous Underwater Vehicle (AUV) is one of the main factors that determine its overall compressibility and drag. The hull as well, acting as a pressure vessel, is the most essential part that will allow the vehicle to accomplish deep diving. In order to minimize the total weight and volume of the vehicle, many modern vehicles benefit from composite materials rather than conventional materials. Moisture absorption, known as the hygral effect, has a significant impact on the mechanical properties of the material. Not to mention it also has an effect on the buoyancy of the vehicle, since the overall weight changes. This paper characterizes the seawater absorption and diffusivity of Carbon Fiber Reinforced Epoxy (CFRE) pressure hull samples in ambient conditions at sea level and at 1000m depth. The tests were performed using seawater taken from Oslo fjord in Norway and using a pressure vessel. Twelve specimens, all manufactured from CFRE using filament winding technique, were tested in both conditions, and the moisture absorption curve is compared. Periodic gravimetric measurements were taken, where the equilibrium state was reached after approximately 65 days at sea level and after 35 days at 1000m depth. The results showed that moisture diffusivity for composites used in underwater applications should be defined by both pressure and temperature since it changes with respect to submersion depth.