## 1 Introduction  
Underwater Wireless Sensor Networks (UWSNs) are comprised of a sensor or a group of sensors that are deployed inside water to sense and explore the deep underwater environments [1]. These sensors communicate wirelessly, with each other and with the central base station where information of different sensors is collected for underwater applications [2]. There are numerous UWSN applications [3] such as underwater monitoring applications (e.g., water quality monitoring, habitat monitoring and deep oceanic exploration), underwater disaster recovery applications (e.g., natural disasters and man-made disasters), underwater military applications (e.g., mines, submarine and surveillance applications), assisted navigation and underwater sports applications. Despite of many UWSN applications, the harsh environment of the underwater environment makes it difficult to design a robust and a reliable network. This is because the underwater environment acts as a channel, and wireless communication through underwater channels has always been extremely challenging. The intricacies presented by the underwater channel are far greater than the problems experienced in terrestrial environment. The prime reason of such intricacies are physical characteristics of underwater channels that tends to allow only restricted frequency bands [4; 5], such as acoustic, electromagnetic (EM) and optical bands. To design an efficient and robust UWSN, the characteristics of underwater channels are of prime importance. The study of underwater physical characteristics (e.g., electro-magnetic properties, propagation and absorption loss) will help to highlight the possible effects of on the operating frequencies and design an adaptable UWSN [4]. Therefore, it is important to discuss the underwater channel characteristics in detail and to know the possible effects of channel characteristics on different operating high-frequency bands. There are different kinds of communication systems for underwater environments. Underwater Acoustic Communication Systems (UW-ACSs) utilize low electro-mechanical frequencies (few Hz) such as acoustics for information transmission from one point to another [6]. Since the frequencies are low (large wave lengths), they are suitable for long distance communication such as ocean depth monitoring and exploration [2; 7]. Acoustic waves get severely attenuated by ambient noise and high water turbidity. Generally, the propagation speed of UW-ACS is approximately 1500 m/s, which results in high propagation delays and Bit Error Rate (BER) [8]. Therefore, for real-time applications, UW-ACSs are not considered suitable. Unmanned underwater vehicles (AUVs), buoys and underwater nodes communicate each other using acoustic signals. On the other hand, Underwater Optical Communication Systems (UW-OCSs) use optical frequencies for information transmission [9]. These are typically very high frequencies of the order of THz. UW-OCSs can be used for shallow water applications specifically at less distances (few meters) as light is attenuated highly at greater distances inside water [9]. Optical laser lights are not preferred for long distance applications because UW-OCS requires accurate Line of Sight (LOS), which is extremely difficult to establish and maintain [6]. AUVs and surface stations may communicate over optical links. Underwater Radio Wave Communication Systems (UW-RCSs) operate on radio waves in which the frequencies are generally of the order from kHz to GHz [2]. As compared to UW-ACSs, the UW-RCSs have the luxury of high bandwidth, high data rates and low propagation delays [10]. Generally, radio wave communication is considered for over the water communication between surface stations and buoys, and on shore infra-structure and anchored underwater nodes may communicate over radio links. Furthermore, UW-RCSs can be used for short distance real-time underwater applications and mostly used in shallow water applications [3] as the utilized radio waves do not penetrate deep in water [2].