In order to address the issues of high friction loss, localized temperature rise concentration, and sealing failure risk in mechanical seals during long-term operation under conditions of medium rotational speed, heavy load, and varying medium pressure for the output shaft of an underwater manipulator"s mechanical cutting arm actuator, a wedge-shaped spiral groove end face seal structure was proposed. A mixed lubrication model was used to systematically investigate the effects of groove parameters, roughness, and operating conditions on the hydrodynamic pressure effect, leakage characteristics, and friction performance of the seal. The hydrodynamic performance and temperature variations of the wedge-shaped spiral groove and the constant-depth spiral groove were compared. The results show that the hydrodynamic load-carrying capacity of the liquid film increases with groove depth, but the improvement tends to saturate when the depth exceeds 3 μm. Both leakage rate and load-carrying capacity increase linearly with the number of grooves. The spiral angle exerts a nonlinear influence on the hydrodynamic effect and leakage rate. As roughness increases, the friction coefficient rises, and the lubrication state transitions from fluid lubrication to boundary lubrication. It is concluded that, compared with the constant-depth groove, the wedge-shaped spiral groove reduces the load-carrying capacity of the liquid film by 79.4%, alleviates heat concentration, and achieves a mixed friction effect. This study provides a reference for the structural design of mechanical seals in underwater equipment.