Is prayer shortening applicable when traveling by ship on the sea?

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If a person is traveling on a ship at sea, the ruling is the same as for land travel. If the intended journey distance reaches the established measure permitting shortening, Qasr is permitted, regardless of whether it is covered quickly or slowly, as the distance is the criterion. If a traveler doubts whether the journey distance permits Qasr, shortening is not permissible, as the default ruling is the completion of prayer, which is not negated by doubt. If one shortens prayer under such doubt, the prayer is invalid, analogous to praying when doubting the entry of the prescribed time.