The seakeeping performance of a ship is one of the primary characteristics to be considered during the design phase to determine the ship’s response in irregular sea conditions. The ship’s main geometric parameters must be selected in the very early stages of design as they influence the seakeeping performance, ensuring compliance with the operational conditions specified by the shipowner. In particular, for cruise ships, it is essential to evaluate the vertical motions to provide comfort for the passengers. It is intended to propose a statistical methodology for calculating the vertical motions of a cruise ship without employing complex theories such as strip theory, which are dependent on hull forms that are still unknown during the concept design phase. The methodology is based on calculating the Root Mean Square (RMS) of heave and pitch motions in various sea states and ship speeds through Multiple Linear Regressions (MLR), using a database of cruise ships created with the design of experiment (DoE) techniques by varying significant geometric parameters affecting seakeeping performance.