## 2 Background
### Internet of Underwater Things  
The IoT is revolutionizing society through applications such as smart cities, precision agriculture, smart healthcare, and more [55; 56]. It is defined as an ecosystem that interconnects a physical device to other physical devices or things across the world, which are interconnected through the Internet. The IoT can play a vital role in several activities/applications in the marine industry as well. The continuous improvement of IoT technology in marine applications has led to the development of a new class known as the Internet of Underwater Things (IoUT) [16; 57]. The IoUT is described as the interconnection of smart intelligent devices that are either fixed or mobile under the water's surface. In recent years, several underwater smart objects have been developed, of which the Autonomous Surface Vehicles (ASVs) [58], Autonomous Underwater Vehicles (AUVs) [59], and Remotely Operated underwater Vehicles (ROVs) [60] are prominent smart devices for exploring the underwater resources.  
The IoUT has some similarities with the terrestrial counterpart the IoT such as structure and function. However, it also has different characteristics [18] when compared with the terrestrial IoT such as:  
1. Different communication technologies;
2. Different tracking methodologies;
3. Difficulty in recharging the battery;
4. Different energy harvesting technologies;
5. Different network density;
6. Different localization techniques.  
The IoUT ecosystem is deployed with an enormous number of intelligent smart objects. As a consequence, the growing demand for device communication and networking with these devices brings a new challenge to this ecosystem. The intrinsic component requiredfor the device communication is the medium or channel through which the information is shared among the smart objects within the network [24]. The UWSN is one of the existing network systems for underwater communication, which is used in several recent IoUT projects [61; 62; 63]. UWSN systems are deployed with numerous underwater sensors mounted with acoustic modems, which are responsible for collecting further significant information such as water temperature, chemical components, pressure, and so on. The collected data are forwarded to different agents on the water surface called the sinks, which are designed with acoustic and radio modems. The sink transfers the information received from the sensor node (acoustic modem) to the remote monitoring center (radio modem) [64].  
With the advancement of the IoUT, several researchers are actively exploring underwater research to interact with the communication devices, thus enabling smart sea technology. As a result, a significant number of IoUT sensors act not only as data collection points, but also facilitate the functioning of other underwater components such as cameras, hydrophones, actuators, and others. Among these, a few remarkable sensors were discussed in [19]. AUVs use acoustic sensors to measure the navigation path and positioning of the smart objects. To detect whether the smart objects are present or not, proximity sensors are used to find the nearby objects by sending electrical signals. Pressure sensors are utilized to determine the pressure level, which is helpful for environmental monitoring applications. Ocean researchers use this significant information generated by the sensors in the IoUT to carefully investigate the unexplored parts of ocean life and underwater resources.