In this study, we investigate underwater energy harvesting from torsional vibration of a patterned ionic polymer metal composite (IPMC). The IPMC design consists of a rectangular polymer strip with patterned electrodes to split the top and bottom surfaces in two equal pairs. We focus on harmonic base excitation of an IPMC, which is modeled as a slender beam with thin cross section vibrating in a viscous fluid. Torsional vibrations with large-amplitude are described using a complex hydro-dynamic function considering nonlinear hydrodynamic damping from the surrounding fluid. Along with the theoretical beam model, an electromechanical model is utilized to predict the IPMC electrical response from the torsional deformation of the beam. The integration of both models allows to predict the output voltage of the IPMC from the knowledge of the frequency and amplitude of base excitation. The theoretical predictions are validated against experiments.