This paper describes a study of the influence of the water on deck on the dynamical behavior of an offshore supply vessel with a large open aft deck. The deck under certain load and sea conditions can become partially or totally immersed. This study focuses on the roll motion that can be of large amplitude and therefore has implications and risks for the ships safety. The Glimms method is used to model the three-dimensional flow of shallow water on the deck. A parametric study is made in order to show the effect of the relevant parameters. It is shown that the water on deck has a significant influence on the ships dynamical response.