Abstract: This study aims to investigate the shear damage and failure characteristics of intact marble under high confining pressure in both dry and saturated states, and further analyzes the softening effect of water on the shear mechanical properties of marble. In hydraulic engineering, deep underground projects, and complex geological settings, water is a key factor influencing rock mechanical behavior, playing a crucial role in rock mass stability and engineering safety design. However, existing research has mainly examined the triaxial compression behavior of rocks, with limited attention to damage evolution and mechanical characteristics under triaxial shear conditions. This study adopts a systematic experimental approach to analyze strength evolution, shear failure modes, and roughness characteristics of shear failure surfaces in marble under varying confining pressures, aiming to provide a theoretical basis for long-term stability assessment and safety design of surrounding rock in deep underground engineering. This study was conducted using the Rock Top-50 HT full-stress multi-field coupling testing system on dry and saturated intact marble samples with simple composition and homogeneous structure under confining pressures ranging from 0–50 MPa. During the experiments, shear stress–strain curves were obtained, and the morphology of shear failure fractures was scanned and analyzed. A non-contact three-dimensional surface scanner was also employed to capture the shear failure surfaces and calculate roughness parameters (e.g., Joint Roughness Coefficient (JRC) and root mean square roughness) to assess the influence of confining pressure on surface roughness. Furthermore, the confining pressure influence coefficient was applied to evaluate the regulatory effect of confining pressure on rock shear strength. A brittleness evaluation index was adopted to analyze variations in rock brittleness under different confining pressure conditions. Considering the water softening effect, the study further explores the impact of water on the shear mechanical properties of rock under varying confining pressures. A comprehensive evaluation of shear failure surface roughness parameters was carried out to quantitatively investigate the evolution of rock failure modes. Under confining pressures of 5–50 MPa, the peak shear strength of marble increased by factors of 2.4, 4.5, 8.6, 10, 13, 16.4, and 19.3, respectively, indicating a significant enhancement in shear-bearing capacity with increasing confining pressure. The confining pressure influence coefficient exhibits a negative exponential decline, with a more rapid decrease in the low confining pressure range (≤30 MPa), while under high confining pressures it tends to stabilize. The water softening effect is more pronounced at low confining pressures. As confining pressure increases from 0 MPa to 5, 10, and 20 MPa, the softening coefficients are 0.667, 0.969, 0.956, and 0.985, respectively, indicating that the water-induced softening of marble gradually diminishes with rising confining pressure. At high confining pressures (≥30 MPa), rock pores and microcracks are compressed, reducing the softening effect, which decreases brittleness and enhances plasticity. The shear failure surface displays an uneven morphology, with a few secondary cracks near the main shear plane caused by stress distribution differences. Roughness parameters decrease as confining pressure increases, underscoring its significant influence on shear failure characteristics. Moreover, pre-peak and post-peak brittleness indices initially rise with confining pressure but subsequently decline, with 30 MPa identified as the critical threshold for brittle transition. The undulation and roughness parameters of shear failure surfaces (JRC, Rrms, Ra, etc.) gradually decrease as confining pressure increases, showing that confining pressure strengthens the constraint effect on shear structures and reduces surface irregularity. Meanwhile, JRC values first increase and then stabilize with rising confining pressure, suggesting that under high confining pressures, the roughness characteristics of shear failure surfaces become more pronounced. Through this research, we have drawn the following four conclusions: 1) Confining pressure exerts a significant influence on rock shear strength, and under high confining pressure conditions, the increase in shear strength tends to stabilize. 2) The water softening effect is more pronounced at low confining pressures, while its impact weakens under higher confining pressures. 3) A confining pressure of 30 MPa represents the critical threshold for brittle transition in marble, with brittleness first increasing and then decreasing as confining pressure rises. 4) The roughness of shear failure surfaces gradually decreases with increasing confining pressure, indicating that high confining pressure restricts crack propagation and reduces the effect of roughness on the shear mechanical properties of rock. This study provides a valuable reference for the long-term stability assessment of surrounding rock in deep underground caverns and for the safety design of rock engineering. It also offers new insights into the mechanical behavior of rock under complex geological conditions. In future engineering practice, it is essential to fully consider the combined effects of confining pressure and hydration to optimize rock engineering design, reduce disaster risks, and enhance the stability of hydraulic, hydropower, and deep rock projects.