This paper presents the construction and testing results for a 63.3:1 dual-stage coaxial magnetic gear for use in a marine hydrokinetic generator demonstrator application. The dual-stage magnetic gear is composed of series-connected coaxial magnetic gear. The stage-2 magnetic gear utilizes Halbach magnet arrays on both rotors and has a 9.5:1 gear ratio. The stage-1 magnetic gear utilizes a Halbach rotor on the outer rotor and a flux concentration inner rotor. The stage-1 magnetic gear has a 6.67:1 gear ratio and at the peak torque of 1220Nm the stage-1 MG was shown to be capable of operating with a 268 N·m/L torque density at 45 r/min and with 95% peak efficiency. The dual-stage magnetic gear was shown to be able to operate with a peak torque density of 228.6 N·m/L and with an 87% efficiency at 45 r/min input speed. The measured efficiency was significantly lower than what was calculated, and the increased losses are believed to be primarily due to the electrical losses induced within the magnetic gears conductive support components.