In this study, we investigate the marine aging behavior of 3D-printed polyamide and carbon fiber–reinforced polyamide composites under near-surface conditions. We evaluated marine exposure using both natural sunlight–assisted seawater aging and an accelerated aging methodology combining Xenon-arc weathering with salt-spray testing, with an acceleration factor defined to relate simulated and real marine conditions. The maximum accelerated aging duration of 10 weeks was equivalent to approximately 5.2 years of natural marine exposure. No significant changes in tensile properties of polyamide or carbon fiber–reinforced polyamide were observed, proving that these additively manufactured parts are suitable for long-term, near-surface marine applications. In contrast, rheological analysis showed a decrease in the viscosity of polyamide after accelerated aging, while there was no change in viscosity under the influence of natural sunlight and seawater, suggesting that heat-induced degradation mechanisms were activated during simulated aging but not in the real marine environment. However, comparison with existing literature highlights the large discrepancies in reports on properties of polyamides after marine aging, which shows the complexity of degradation processes and the interaction of multiple mechanisms. With the increasing use of unmanned underwater vehicles and marine drones, accurate prediction of the lifetime of plastic components is essential to minimize waste in the marine environment.