Underwater Compressed Air Energy Storage (UWCAES) offers a scalable solution for storing intermittent renewable energy. It has high volumetric energy density, does not require cushion gas, provides flexible offshore sites, and requires minimal onshore and use. To date, field-scale validation of such systems in real marine environments, especially at practical depths (> 30 m), remains limited. To address this gap, we present the first field trial of a rigid-tank UWCAES system deployed at depths of 30–70 min the Red Sea. The system comprises a 294 L rigid PVC tank with an open bottom designed for air–seawater displacement during repeated charging and discharging cycles. Real-time measurements of pressure, temperature, and flow rate were conducted across different outlet nozzle configurations and storage depths. Experimental results demonstrated stable system operation and effective energy delivery. During discharge, the volumetric flow rate remained nearly constant, indicating consistent power output. Increased storage depth led to higher flow rates, highlighting the role of hydrostatic pressure in enhancing system performance. Despite the expected cooling from adiabatic expansion, the compressed air temperature along the submerged hose sections remained close to the local seawater temperature and increased further in the above-water sections. This temperature profile suggested passive thermal buffering from the surrounding seawater and additional warming from ambient air exposure near the outlet. These findings provide new empirical insights into the thermal dynamics, operational stability, and offshore deployment feasibility of UWCAES systems at practical depths.