Non-linear free-surface phenomena, such as extreme waves and violent sloshing, and their impact on the dynamics response of the containing vessels have long been subjects that could only be studied with experimental methods. Nowadays, computational CFD tools can make a significant contribution to these flow problems. The paper starts with a short overview of the most popular numerical methods to simulate highly non-linear free-surface phenomena, with emphasis on Navier--Stokes methods. Thereafter, it describes the development efforts made in the maritime SAFE-FLOW project and the micro-gravity SloshSat FLEVO project. In particular, the improved Volume-of-Fluid (iVOF) free-surface simulation method ComFlo is presented. Examples of violent fluid dynamics in both application areas are presented. In all cases experimental data are available to validate the calculations.