<p>To address the poor compliance and vulnerability to collision damage in traditional rigid underwater snake robots, a water hydraulic-driven variable-stiffness soft joint is proposed to enhance their safety and compliant operation capabilities. An actuator module was designed based on the antagonistic mechanism between a fluidic actuator and a mechanical spring. Furthermore, a dedicated hydraulic actuation system was developed to satisfy the strict volume and power consumption constraints of the soft joint. Based on geometric mechanics, a mathematical model of the soft joint was established, utilizing an iterative method to decouple its stiffness and posture. A series of experiments were conducted to validate the proposed design and theoretical model. The results indicate that the soft joint achieves a maximum bending angle of 33.8°. The maximum and minimum bending stiffness values are 350 N·m/rad and 28.1 N·m/rad, respectively, demonstrating a significant stiffness variation ratio of 12.55. This soft joint not only meets the application requirements of underwater snake robots but also exhibits potential for other flexible robotic manipulators with stringent power and spatial limitations. Finally, the proposed stiffness-posture integrated control strategy achieves the decoupled regulation of the joint's stiffness and posture, offering new insights into the compliant operation of underwater flexible robots.</p>