Underwater operations—such as marine environmental protection, resource recovery, and seabed exploration—require grippers with high adaptability. Existing rigid and soft grippers are constrained by their inherent material limitations, restricting their manipulation versatility. In this work, we introduce an octopus-inspired underwater gripper with rapidly tunable stiffness, integrated into an upward transport robot designed for efficient underwater object manipulation. Achieving softening in 1.3 s and rigidification in 0.8 s, the gripper demonstrates the shortest stiffness transition time reported to date, substantially advancing rapid and adaptive underwater manipulation. Emulating the octopus’s multimodal grasping strategy, the system can handle a wide range of objects—from light to heavy and soft to rigid—even in cluttered underwater environments. The integrated robot combines active buoyancy control with manipulation to enable continuous grasping and vertical transport of submerged objects. This study offers a robust solution for adaptive underwater manipulation, with potential applications in autonomous marine operations, ecological restoration, and ocean missions.