This paper presents an online estimation based adaptive nonlinear maximum power point tracking (MPPT) controller for a stand-alone permanent magnet synchronous generator (PMSG) based marine current energy conversion system. The proposed control strategy requires no flow sensor and also does not need the parameters of a PMSG. A nonlinear control algorithm for the load side converter to extract maximum power has been proposed to adequately deal with the inherent nonlinearities in the energy conversion system. A Lyapunov based online estimation approach is used to continually estimate the time varying input voltage and the output load resistance of the converter. Detailed simulation results of the proposed nonlinear controller namely adaptive backstepping are presented and fully analyzed. Simulation results demonstrate that the proposed nonlinear controller can incessantly extract maximum power from the ocean current at various flow speeds.