Continuous physiological monitoring is paramount for ensuring the safety of divers in extreme underwater environments, where risks occur such as central nervous system oxygen toxicity (CNS-OT) due to the breathing mixture. Electrodermal activity (EDA) serves as a vital physiomarker for sympathetic nervous system activation; however, its measurement in seawater is hindered by ionic interference. Herein, we report a robust, superhydrophobic-coated EDA sensor fabricated via a scalable injection molding process and a hybrid carnauba wax coating strategy. By integrating dip-coating and spray-coating techniques, we achieved a hierarchical microstructure on a PDMS substrate, resulting in a water contact angle of 158°. The dual-layered coating maintained hydrophobicity after 900 tape-peeling and 50 abrasion cycles, with a square-shaped trench further precluding water ingress at the electrode interface. In underwater human trials involving various physiological and cognitive stressors, the sensor demonstrated high-fidelity acquisition of EDA signals. The data showed high synchronicity with conventional Ag/AgCl electrodes (Pearson correlation > 0.5) and remained stable despite 30 hours of saltwater immersion and dynamic motion artifacts. This study provides a reliable, mass-producible platform for real-time health monitoring in maritime operations, representing a significant advancement in underwater wearable bioelectronics.