In this work, we studied the impact of the buoy geometry on power absorption of a point absorber consisting of a cylindrical buoy. The displacement volume of the buoy is fixed while the radius to draft ratio is adopted to completely define the buoy geometry. A motion constraint that prevents the buoy from jumping out of or being submerged by the water is applied so that the absorbed power is restrained at a reasonable range. The maximum power absorption ability under reactive control has been attained, and the most appropriate buoy geometry has been identified. Passive control is also investigated to find the possible condition where it can be used as an alternative to active control. Results show that the maximum absorbed power approaches the Budal limit at low wave frequencies, and gradually departs from this limit with the increasing wave frequency. The most attractive radius to draft ratio, which shows more absorbed power and wider bandwidth, is around 1.0 and decreases with the increasing wave amplitude. At high wave frequencies, the absorbed power is identically affected by the motion constraint for both passive and reactive control, hence passive control may be an alternative to reactive control in this condition due to simpler implementation.