Ballast water has frequently been used in ships to provide stability and adjust trim, stress, and torsion for optimal steering and propulsion. Numerical simulation of the movement of ballast water and its interaction with the solid walls of operating ships are very difficult for traditional grid-based numerical models. In this paper, the smoothed particle hydrodynamics (SPH) method is applied to simulate water tank sloshing and the movement of ships carrying ballast water in three cases. Numerical results of water tank sloshing are compared with experimental ones. Numerical results of ships indicate that carrying ballast water in several separated small tanks generally makes a ship more stable, but keeping ballast water in one big tank generally makes a ship more unstable.