This study develops a new numerical simulator with ISPH-DEM coupling for rubble mound failure prediction and the other soil-water interactions. A coupled fluid-soil-structure simulator is developed by combining the ISPH for fluid and the DEM for rubble mounds and caisson blocks. The ISPH can simulate seepage and violent flows such as tsunamis. The DEM has been applied for discrete particle and rigid body simulations that include discontinuous deformation, as in the rubble mounds failure and large displacement of the caisson block. Firstly, each rubble is modeled as an idealized spherical DEM particle with the mean diameter of the rubble. The ISPH particle size is assumed to be the same size as the DEM particle. Under these assumptions, the unresolved coupling model between rubble mound particles and fluid, which obtains the interaction through empirical drag force, should be applied. We already proposed the ISPH-DEM coupling simulator in, but it still has instability problems related to the low accuracy in the ISPH simulation. We therefore have proposed a second-order SPH model, SPH(2), that satisfies second-order accuracy in both first- and second-order derivatives. The key idea is based on the LDD particle method proposed by Basic et al. The results confirmed the superiority of SPH(2) over the existing SPH models. In this research, we combine our two new techniques; ISPH-DEM coupling simulator and SPH(2) for more reliable simulator for soil-water interaction problems. After the simple validations with lid-driven cavity flow and Karman’s vortex problems, we shows a better performance of ISPH-DEM simulator with help of the second-order SPH model, SPH(2).