The performance of wave energy converters is significantly affected by buoys. This research studied the force and conversion efficiency of buoys in low wave energy density seas and involved the selection of a cylindrical buoy and a hemispherical buoy, which were applied in a sealed-buoy wave energy converter (SBWEC). The underwater equivalent area was utilized as the variable to investigate the force, pitch response amplitude operator (RAO), and conversion efficiency of the buoys in irregular waves. The results show that the common responses occurred both in the cylindrical and hemispherical buoys. The force of the buoys in a resonance state was distributed in a concentric ring shape and gradually decreased from the inner to the outer areas. The force, pitch RAOs, and conversion efficiency increased with a corresponding increase in the underwater equivalent area. Results determined that owing to the difference in the underwater equivalent area and shape, the force of the cylindrical buoy with a diameter 2 m and 4 m was 194–1455% larger than that of the hemispherical buoy with the same diameter. The pitch RAOs of the cylindrical buoy were 0.6–52.59% larger than that of the hemispherical buoy because of the difference in force. In turn, the average conversion efficiency of the cylindrical buoy was 1.31% greater than that of the hemispherical buoy. The novel SBWEC made the wave energy conversion efficiency of the buoys in low wave energy density seas 0.6–1.4% less than that in high wave energy density seas. The utilization of the underwater equivalent area provides a new method for researching the force and conversion efficiency of buoys.