## 1 Introduction  
Water-bodies cover two-thirds of the Earth's surface. Moreover, from the very beginning of human civilization, people have been choosing water-bodies to live near. The ocean has always played the most important role as a transportation medium, sources of natural resources and host of all marine lives. But hardly 5% of the whole ocean volume has been explored [1]. To inspect the unknown, underwater communication has recently attracted research attention. Traditional systemsfor underwater monitoring have some limitations and this inimical environment is not suitable for humans (due to high underwater pressure) to explore the deep ocean and its expansive areas. This inspires the unmanned exploration of these dicey environments. Neither unmanned exploration nor distant real-time monitoring is possible without deploying Underwater Wireless Sensor Networks (UWSN). Consequently, UWSN has recently drawn the interest of researchers. This underwater wireless sensor network is different from the traditional wired and wireless terrestrial sensor networks [2; 3]. UWSN is composed of a different number of sensor nodes which are deployed at different heights/depths of the sea volume. The number of sensor nodes depends on the volume of the sea that is to be covered and other factors such as the transmission range of the nodes, the desired performance of the network, and so forth. These sensor nodes at different levels of the sea collect the necessary data and eventually send it to the data sinks at the sea surface. These data sinks send the collected data to the nearest terrestrial network by satellite or Radio Frequency (RF) communication. The data transmission from the surface sink to the terrestrial network follows the traditional protocols for terrestrial communication. The terrestrial communication system uses radio frequency (RF) as their medium of communications and traditional network/communication protocols (e.g., Transmission Control Protocol (TCP)) which are not feasible underwater due to multiple reasons including high attenuation and low transmission range [4]. Wireless sensor networking using acoustic media and channels turn out to be the most realistic solution for underwater communications. However, the deployment of the network, the architecture of the network, localization, and reliability of underwater wireless sensor networks (UWSN) are not similar to those of the terrestrial system when it comes to acoustic media and channel [5]. In comparison to the terrestrial sensor network, UWSN is very sensitive to its three dimensional (3D) nature due to high wave movements. The aim of the UWSN is to collect real-time data and locate from where data is being collected. This helps to warn about natural hazards precisely and save thousands of lives. The UWSN also collects data from the marine environment, detects underwater mineral mines, and so forth. Real-life applications of UWSN also include seismic monitoring, ocean sampling, vision-based underwater mine searches, ecological monitoring, monitoring of underwater marine cables, pipelines of gas and oil, and so forth. However, for these applications, the location of the source of the data needs to be known. Because of this, the study of underwater localization is important. As an acoustic medium is used underwater, (Global Positioning System) GPS technology is not feasible in UWSN. Hence, the localization techniques are different in underwater communication systems [6; 7]. One of the challenging factors of UWSN is the reliability of the network. As the underwater condition is very hazardous and prone to sudden changes, it is difficult to maintain the reliability of the network constantly. Keeping in mind the challenging factors, researchers have proposed different routing protocols for UWSN. Most of these routing protocols using acoustic medium are either single-hop or multi-hop, or, either clustering or clustering-multi-hop communication. Here, different routing protocols emphasized in different factors of the network.