[Objective] To address the stringent requirements of high reliability, strong robustness, and smooth full-speed-range control for underwater thruster drive systems in complex environments, a hybrid high-frequency square-wave injection method/sliding mode observer (HFI/SMO) sensorless control strategy is proposed. [Methods] A dual-mode switching strategy of HFI/SMO was adopted: the HFI was used in the zero/low-speed range, and the SMO based on a phase-locked loop was employed in the medium/high-speed range. Smooth transition was achieved through a weight-switching module. The dynamic and static performance of the proposed algorithm, as well as its engineering practicability, were verified through simulations. [Results] The simulation results demonstrated that the proposed hybrid HFI/SMO control strategy maintained high-precision control across the full speed range. At low speed (200 r/min), the speed fluctuation was maintained within ±5 r/min (±2.5% error). At high speed (3 000 r/min), the fluctuation was constrained within ±8 r/min (±0.27% error), achieving high-precision speed regulation standards. During acceleration from 0 to 3 000 r/min, the maximum tracking error was suppressed below 40 r/min without overshoot, exhibiting superior dynamic tracking performance. [Conclusion] The proposed hybrid HFI/SMO control strategy meets the precise speed regulation requirements of underwater thrusters, providing a practical solution for high-performance sensorless drives.