The violent free-surface motions interacting with floating vessels containing inner liquid tanks are investigated by using the newly developed Moving Particle Semi-implicit (MPS) method for 2-dimensional incompressible flow simulation. In the present numerical examples, many efficient and robust algorithms have been developed to improve the overall quality and efficiency in solving various highly nonlinear free-surface problems and evaluating impact pressures compared to the original method proposed by Koshizuka and Oka (1996). For illustration, the improved MPS method is applied to the simulation of nonlinear floating-body motions, violent sloshing motions and corresponding impact loads, and vessel motions with inner liquid tanks. It is seen that the roll amplitudes can be significantly reduced due to the presence of the sloshing tank when the excitation frequencies are away from the lowest sloshing natural frequencies. The developed numerical tools can be used to the study of vessel motions with liquid cargo or design of passive anti-rolling devices.