###### Abstract  
The Internet of Underwater Things (IoUT) is a novel class of Internet of Things (IoT), and is defined as the network of smart interconnected underwater objects. IoUT is expected to enable various practical applications, such as environmental monitoring, underwater exploration, and disaster prevention. With these applications, IoUT is regarded as one of the potential technologies toward developing smart cities. To support the concept of IoUT, Underwater Wireless Sensor Networks (UWSNs) have emerged as a promising network system. UWSNs are different from the traditional Territorial Wireless Sensor Networks (TWSNs), and have several unique properties, such as long propagation delay, narrow bandwidth, and low reliability. These unique properties would be great challenges for IoUT. In this paper, we provide a comprehensive study of IoUT, and the main contributions of this paper are threefold: (1) we introduce and classify the practical underwater applications that can highlight the importance of IoUT; (2) we point out the differences between UWSNs and traditional TWSNs, and these differences are the main challenges for IoUT; and (3) we investigate and evaluate the channel models, which are the technical core for designing reliable communication protocols on IoUT. Internet of Things (IoT); Internet of Underwater Things (IoUT); smart city; Underwater Wireless Sensor Networks (UWSN); wireless sensor networks (WSN) +
Footnote †: journal: Sensors  
0000-0002-4000-0000-0000-0000-00000-0000-0000-00000-0000-00000-00000-00000-00000-00000-00000-00000-00000-00000-00000-00000-00000-00000-00000-00000-000000-000000-00000-000000-000000-00000-00000-000000-000000-000 Second, IoUT is expected to enable numerous applications in smart cities. Third, the underwater communication and equipment waterproofing technologies are at rather mature stage. Accordingly, this is an appropriate time to research IoUT. For IoUT, Underwater Wireless Sensor Networks (UWSNs) have been regarded as a promising network system. Figure 1 shows the network architecture of UWSN. Note that readers interested in the UWSN equipment used in real-world projects are referred to [8; 9; 10; 11]. As shown in Figure 1, UWSNs consist of several components. The main components are underwater sensors. In UWSNs, the sensors are the nodes with acoustic modems, and are distributed in either shallow or deep water. Each sensor node can sense (different sensors can sense different environmental information, such as the water quality, pressure, temperature, metal, and chemical and biological elements), relay, and forward data. The data should be transferred to the essential component(s) on the surface of the water, called sink(s). Sinks are the nodes with both acoustic and radio modems. Note that sinks can be buoys, ships, or ASVs. When data arrive at sinks (through acoustic channels), the sinks will forward data to the remote monitoring center (through radio channels). The monitoring center is often on the seashore, and is responsible for monitoring the water areas. The monitoring center collects, analyzes, and deals with the information from the water areas. In addition, the AUs in the deep water can also be the optional components of a UWSN. The AUs can help collect and forward data. With the above useful and practical components, UWSNs are expected to enable a diverse range of IoUT applications, such as environmental monitoring, underwater exploration, and disaster prevention applications. However, the properties of UWSNs are different from those of the traditional Territorial Wireless Sensor Networks (TWSNs). The unique properties of UWSN, such as long propagation delay, narrow bandwidth, and low reliability, would be great challenges for IoUT. In the literature, many studies have concentrated on tackling the problems of the end-to-end delay, lack of bandwidth, and dynamic topology, but few studies have looked at the more fundamental problem of reliability.