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Ocean current energy harvester is a promising infrastructure to achieve self-powered marine wireless sensing system. This study proposes and investigates an underwater flag-like Triboelectric Nanogenerator (UF-TENG) based on the flow-induced vibration. The UF-TENG consists of two conductive ink-coated polyethylene terephthalate membranes and one strip of poly tetra fluoroethylene membrane with their edges sealed up by a waterproof PTFE tape. In this way, the triboelectric layers are prevented from contacting water. It is verified that the vortex street induced by a cylinder enhances the vibration of the UF-TENG. The low velocity startup enables the UF-TENG to harvest extremely low-velocity ocean current energy, e.g., presently achieving 0.133 m/s as the critical startup velocity. The proposed UF-TENG has a simpler structure and better low velocity performance when comparing with electromagnetic generators and piezoelectric generators. Parametric studies are conducted to evaluate the influence of geometrical parameters of the UF-TENG on the vibration behavior and output performance. It is demonstrated that the underwater electrical appliance can be powered by parallel UF-TENG units. The present UF-TENG is a more cost-effective and more accessible hydroelectric technology to utilize the renewable ocean current for powering sensors or micro electric appliances in Internet of Things.