This thesis aims to investigate the performance of a horizontal water turbine in ocean current. The design of the water turbine is based on the Blade Element Momentum theory to begin with. As the water current flows past a single turbine, the water inflow velocity and the rotational speed are the parameters to be investigated. Furthermore, the interaction of more than two turbines due to the relative distance is also discussed. The relative distance encompasses both the front and the back. The results show that the water inflow velocity and the turbine rotational speed influence the performance of the turbine. When two turbines function simultaneously, the flow field is different from the one of a single turbine and thus influences the performance of the other turbines in the vicinity. Lastly, the site arrangement of three turbines is discussed, and it is revealed that a proper arrangement can enhance the performance of the turbines.