Polymer composites in marine structures that operate under seawater environment may be seriously affected, reducing durability estimates. This work aims at evaluating the effect of seawater exposure at 80 °C for 7–28 days on filament-wound glass fiber/epoxy composite cylinders partially cured by passing saturated steam through them just after winding seeking a faster curing route. The winding angle is varied (±55, ±65 and ± 75) and some of the cylinders are later post-cured for comparison. The unaged partially cured specimens do not reach complete curing, with a glass transition temperature (Tg) of 132 °C, below the Tg for the other samples (154–159 °C). Fully cured cylinders present mechanical properties slightly higher than partially cured ones. Moreover, aging in seawater for 7 days enhances the cross-linking degree of epoxy, with a positive effect on both hoop tensile strength and stiffness. Aging is, however, not critical for the radial compressive properties.