A nonlinear oscillator, using a cubic-spring restoring function with high-deformation stiffening, is introduced and studied experimentally to improve passively the harnessed marine hydrokinetic power using Flow Induced Vibrations (FIVs) of a cylinder. In this research, the FIV of a single, rigid, circular cylinder on elastic end-supports is tested for Reynolds number 30,000 ≤ Re ≤ 120,000. Damping, cubic stiffness, and flow-velocity are used as parameters. Selective roughness is applied to enhance FIV and increase the hydrokinetic energy converted by the oscillator. The second generation of the digital, virtual spring-damping system Vck, developed in the Marine Renewable Energy Laboratory (MRELab), enables embedded computer-controlled change of the functions and values of viscous damping and spring stiffness. Cubic modeling of the oscillator stiffness in parametric form is thus realized and tested. Experimental results for amplitude response, frequency response, energy harvesting, efficiency and instantaneous energy of the converter are presented and discussed. All experiments are conducted in the Low Turbulence Free Surface Water (LTFSW) Channel of the MRELab of the University of Michigan. The main conclusions are: (1) The cubic stiffness function is an effective way to raise the harnessed efficiency over a wider range of flow velocities. (2) At lower flow speed (upper and lower VIV branches), the harnessed power increases as the nonlinearity increases. A strongly nonlinear system exhibits a 100% increase in harnessed energy in this region. (3) At a higher flow speed (galloping), the cubic nonlinearity benefits the harnessed power in two ways because the natural frequency of the oscillator in water (fn,water) depends on the amplitude of oscillation. At low harness damping, the amplitude increases resulting in higher fn,water thus enhancing the harnessed power. At high harness damping, the harnessed power increases regardless of fn,water.