Underwater monitoring and exploration have enhanced significantly due to the wide adoption of Internet of Underwater Things (IoUT). However, IoUT implementation is limited by batteries that require frequent replacement, which is costly and unfeasible due to the hostile aquatic environment. Therefore, it is crucial to implement an energy-efficient solution that maximizes the lifetime of IoUT devices, and hence reduce the overall cost of the system. Therefore, we propose the use of ondemand wake-up radios, where IoUT devices are activated upon receiving a wake-up call from the deployed surface buoys via different communication approaches (i.e., acoustic, optical, and magnetic induction). To analyze the performance of the proposed wake-up scheme, we derive an analytical-based solution for both the success and false wake-up probabilities for the different underwater communication techniques. Finally, we evaluate the performance of the proposed scheme under different design parameters and highlight the wake-up-based solution’s gain.