Self-noise is problematic in many underwater applications, since it interferes with the performance of onboard sensors. To reduce self-noise, experiments investigated using composite materials for cylinders subjected to hydrostatic pressure loading. This paper presents the test procedure and results of modal analysis testing conducted in-air and underwater on some composite cylinders. The in-air tests used impact force excitation, whereas the underwater tests used sound excitation. The results include resonant frequencies, damping factors, and mode shapes at resonance. In order to ensure a valid comparison, the natural frequencies and damping factors are compared on a mode-by-mode basis. Despite the increased complexity of the underwater experiments, it is shown that the in-air and underwater levels of damping observed at relatively low frequencies are comparable. Further, an appropriate choice of composite materials can increase damping by at least an order of magnitude over conventional metals. The damping can additionally be increased by using a double-hollow core or triple-shell configuration, with reinforced polyurethane ribs. The paper discusses this new reinforced polyurethane as a structural component of the cylinder, and proposes various mechanisms of damping that this material form provides.