The paper presents a methodology to calculate the seakeeping performance of ships, which is given as an operability index, and discusses the sensitivity of the results to the use of different seakeeping criteria. The calculation of the operability index, which represents the percentage of time during which the ship is operational, depends on the wave climate of the ocean area where the ship operates, the dynamic response of the ship to the waves, and the ship mission. The relation between the ship operability and the mission characteristics is established through the seakeeping criteria. The calculation of operability indexes and the sensitivity analysis are carried out for a containership operating in the North Atlantique between Europe and the United states, and a fishing vessel operating near the Portuguese west coast. These are two ships with different mission profiles, which permits assessment of the sensitivity of the estimated operability index to different ship types.