RoPax vessels are designed for transporting passengers, vehicles and cargo, ensuring comfort and safety. The absence of transversal subdivisions poses significant risks in case of garage deck flooding, affecting ship stability and deck structure loads. This paper introduces a fast numerical method to study the drainage capacity of a RoPax vessel’s garage deck using a passive drainage system (scuppers only) during firefighting scenarios. The drainage system’s effectiveness is simulated in calm water and irregular waves using a hybrid non-linear model, considering two scupper configurations with varying dimensions and positions. Results suggest that placing more scuppers at the stern improves drainage efficiency. The analysis of the results can provide general support for the design of passive drainage systems.