Development of an ocean wave energy-harvesting device that can be used as a renewable energy source for ocean monitoring systems. The core technology is a mass-spring based high efficiency, low frequency linear generator that was integrated to a spar buoy to form a robust unit. Its dynamic sensitivity is improved by the use of a near zero friction liquid bearing. Through extensive theoretical modeling and wave tank testing, we studied the effect of enhanced heave motion due to resonance with waves and applied it to improve the energy harvesting efficiency. We also developed a computer simulation model that can accurately predict the device performance driven by a single frequency, sinusoidal forcing function. Full-scale models were fabricated and field tested in non-random waves. Power output was measure from 1.6 W to more than 6 W, depending on the device size, oscillator characteristic and wave conditions. Finally, we carried out extensive studies to build and characterize several small underwater motion energy harvesters of different designs. A complete performance metrics was established.