Numerical simulation of natural hazards such as seepage-induced internal erosion, debris flows, and mixed solid–fluid interactions requires flexible and robust coupling strategies between fluid and solid phases. In particle-based methods, coupling approaches are commonly classified into resolved and unresolved models, depending on whether solid objects are explicitly resolved by the fluid discretization or represented through subgrid-scale interactions. Resolved models provide detailed fluid–solid interactions but become computationally expensive for fine particles, whereas unresolved models are efficient but often struggle to capture localized failure mechanisms. This study presents recent advances in resolved and unresolved SPH–DEM coupling frameworks for solid–fluid interaction problems, using a fluid solver enhanced by a second-order accurate SPH(2) formulation. For resolved coupling, a fluid–rigid body interaction framework based on the fictitious domain method is developed, including weakly coupled and partitioned iterative strong coupling schemes. These formulations enable robust and accurate simulations of floating and moving bodies under strong fluid–structure interaction effects, with improved stability and accuracy for lightweight bodies subject to added-mass effects. For unresolved coupling, a particle-based seepage flow model is integrated with the SPH(2)-based fluid formulation, enabling three-dimensional simulations of free-surface flow, seepage flow, and solid–fluid mixture flow within a unified framework. This approach allows seamless transition between surface and subsurface flow regimes while maintaining computational efficiency. Numerical examples demonstrate the respective capabilities and limitations of resolved and unresolved coupling approaches, highlighting the importance of appropriate coupling strategies and high-accuracy fluid discretization for modeling natural hazards involving gap-graded materials and complex solid–fluid interactions.