The complex underwater environment presents numerous challenges for the design of soft grippers, which often suffer from limited load capacity, poor stability, low portability, and imprecise control. This paper proposes a novel rigid-soft hybrid gripper specifically designed for underwater use. The gripper's finger is constructed from silicone, reinforced with a multi-link rigid exoskeleton on the outside, and actuated by tendons. This design provides three key advantages: compliance (capable of handling fragile objects such as a piece of tofu), heavy lifting (demonstrated by lifting an 80 kg barbell with three fingers), and precise, stable operation (the hybrid gripper maintains its shape despite water flow disturbances). Additionally, the gripper is compact and lightweight, with the driving system powered by just four 23g servo motors, making it easy to mount on various underwater robots. To enable precise control, both specialized kinematic and mechanics models were developed, allowing accurate predictions of the relationships among tendon displacement, exoskeleton deformation, soft material deformation, and tendon tension. This study thoroughly considers the challenges of underwater environments, offering new insights for advancing the field of underwater soft grasping.