Abstract: In view of the problems in underwater concrete strength detection, such as the complex retrofitting process of testing instruments, immature waterproof modification technology for digital rebound hammers, and high risks associated with manual detection, this study proposes a waterproof retrofitting method for rebound hammers and ultrasonic detectors based on 3D printing technology. A device was developed to mount the testing instruments onto underwater robots. Using the robot equipped with the waterproof-retrofitted instruments, rebound values and ultrasonic wave velocities were measured on standard test blocks in a static water environment. Outlier rejection was performed on the test data, and the least squares method was employed to fit correlation curves between the rebound values, wave velocities, and compressive strength of the test blocks. Three specialized underwater strength prediction curves were established based on the ultrasonic method, rebound method, and combined ultrasonic-rebound method. The accuracy of the curves and the feasibility of underwater robot-assisted detection were verified, demonstrating their applicability to underwater concrete strength testing.