Plug-in Hybrid Electric Recreactional Boat (PHERB) is a new innovation of conventional boat for water transportation. In the PHERB powertrain, the main power source to drive the boat is the electric machine (EM). The primary energy source of EM is the battery. Battery is an important power energy supplier to PHERB and has two main conditions such as charging and discharging conditions. In this paper, the optimal design of the battery in a discharging condition is reported. The battery model is developed in MATLAB/Simulink environment together with a closed-loop feedback PI controller. By using a power demand curve as a reference for the model, the optimal performance of the discharging battery is obtained by using the genetic algorithm optimization. The results of optimal control parameters of the system are compared with the trial-and-error method. It has been found that the proposed optimal system design can improve the discharged battery's performance significantly.