In this paper, a novel electromagnetic pendulum energy harvester with a counterweight is designed to harvest low frequency vibration from ocean waves for unmanned surface vehicles. This design is the first of its kind, allowing the natural resonant frequency of the pendulum to be reduced without increasing its length, thereby maintaining a high power output from the energy harvester at lower frequencies than previously possible with pendulums of the same size. Implementing a novel mechanical rotation rectifier (MRR) system for a high energy conversion efficiency, this counterweight pendulum energy harvester can provide multi-watt-level power at frequencies lower than 1 Hz, with a primary pendulum arm length of just 195mm. When actuated at 0.1 g rms, the pendulum energy harvester with a counterweight produced electrical power of 0.997 W at 0.75 Hz, compared to 0.168 W without the counterweight. The average normalised power output of the system at this frequency is 95.8 W/g2, corresponding to a power density of 6.11 W/g2/kg. Testing of a range of configurations of the pendulum mass and counterweight shows a clear linear relationship between the ratio of lengths of the pendulum arms and the reduction of the natural frequency of the system. This demonstrates empirically that this device is capable of operating under conditions in which existing energy harvesters are unable to provide adequate power, and therefore provides a significant development in energy harvesting in an ultra-low frequency marine environment.