What are the three factors important in causing currents?

Wind (the biggest factor in surface currents), Water density, and the Coriolis effect ("an effect whereby a mass moving in a rotating system experiences a force (the Coriolis Force)  acting perpendicular to the direction of motion and to the axis of rotation. On the earth, the effect tends to deflect moving objects to the right in the northern hemisphere and to the left in the southern and is important in the formation of cyclonic weather systems.")