The collapse of natural ground and earth structures caused by underground and surface water flow can result in significant geotechnical disasters such as breach of river levees, debris flows, and submarine landslides. Numerical simulation is a useful tool for predicting such behaviors and assessing risks. However, methods that adequately account for seepage, free-surface water flow, and ground failure accompanied by large deformation under water interactions are still under development. This study proposes a coupled approach combining the implicit Material Point Method (MPM) [1] for large deformation analysis of ground and the Volume of Fluid (VOF) method [2] for free-surface flow analysis. The momentum conservation of the soil-water two phase mixture is solved using MPM, while water flow inside and outside porous media is analysed using the VOF method. The Darcy-Brinkman equation serves as the governing equation, and the finite difference method is employed for temporal and spatial discretization. The fractional step method is employed to couple the Darcy-Brinkman equation with the continuity equation, and to ensure stability in low-permeability grounds, an algorithm implicitly solving soil-water interaction (drag force) term was also proposed. The validation of the proposed method was confirmed through analyses of 1-D and 2-D boiling phenomena of saturated ground. Furthermore, the MPM-VOF was applied to simulate a landslide on a submarine slope containing a thin impermeable layer. The simulation includes the initiation process due to the rise of excess pore water pressure, and the post-failure phase with a large deformation. The results demonstrated that the presence of the impermeable layer plays a crucial role in the accumulation of excess pore water pressure and reduction of the effective stress in the ground.