Ocean wave energy is a promising energy source for large-scale exploitation owing to its abundant reserve and renewability. An effort is underway to develop a generator that can adapt to the ultra-low frequency of ocean wave vibration. Here, we designed a cylindrical triboelectric nanogenerator with an internal swing structure for effective water wave energy harvesting. Based on the supporting effect of the bearing component, the rotary dielectric films can be suspended over the stator electrodes, rather than coming into direct contact with them. Benefiting from the largely reduced resistance and continuous swing of an internal rotary component, the fabricated triboelectric nanogenerator can operate for about 85 s and produce over one thousand current pulses after one water wave excitation. Being agitated by a 0.033 Hz water wave (during a period of 30 s), the optimized cylindrical triboelectric nanogenerator can produce a peak power density of 231.6 mW·m⁻³ and an average one of 39.8 mW·m⁻³. Successful demonstrations of powering portable electronic devices and iron material corrosion inhibition under such simulated excitation indicate the robust capability for ultra-low-frequency water wave energy conversion. This triboelectric nanogenerator offers a new vision for efficient ultra-low-frequency wave energy harvesting with the possibility of large-scale blue energy.