In this work, we present an analytic and numerical model for energy harvesting of an underwater acoustic spherical sensor. The energy harvester is composed of two half-spheres made of Plexiglas with a PZ26 piezoelectric ring fixed between them. For the analytical model, the piezoelectric ring was modeled using primary thickness and radial vibration modes. For each mode, the ring is described by an equivalent electromechanical circuit interconnecting the mechanical part (forces and velocities) and electrical parts (voltage and current). Consideration of two electromechanical circuits allowed simulating total recovery voltage response. In order to validate the analytical model, we have developed a finite elements model with Comsol Multiphysics. The values of voltage response and harvested magnitude obtained with the analytical model have been confirmed by the results of numerical simulation.