The purpose of this communication is to discuss the simulation of a free surface compressible flow between two fluids, typically air and water. We use a two fluid model with the same velocity, pressure and temperature for both phases. In such a model, the free surface becomes a thin three dimensional zone. The present model has at least three advantages: (i) the free-surface treatment is completely implicit; (ii) it can naturally handle wave breaking and other topological changes in the flow; (iii) one can easily vary the Equation of States (EOS) of each fluid (in principle, one can even consider tabulated EOS). Moreover, our model is unconditionally hyperbolic for reasonable EOS. First, we present the physical context of our study. Then, we introduce the governing equations and we give some rationales on the limit of this model to the classical free surface model. Finally, we present our numerical method based on a flux scheme which is, in particular, constructed to model accurately impacts of waves on walls. Since our code is designed for unstructured meshes, it can easily treat complex geometries (for example, liquified natural gas carrier tank). This communication will conclude with the presentation of different simulation results on the sloshing of a liquid in a closed container.