## 1 Introduction  
The planet Earth, on which we live our lives, consists of 70% water. Whereas the oceans hold more than 90% of total water. This much quantity shows the importance of the water medium. To explore the underwater medium for getting and sharing important information, a network is deployed in a specific region. Information transmission using Underwater Wireless Sensor Networks (UWSNs) is one of the emerging technologies and is used for the betterment of ocean observation systems. Applications of UWSNs range from aquaculture to oil industry; instrument monitoring to climate recording; pollution control to predictions on natural disasters; search and survey purposes to submarine purposes. The sensor node in an UWSN acquires the desired information and transmit towards the next forwarder node which is closer to the sink (Sink: This word is alternatively used as sink node, sonobuoy, destined node and destination node) [1]. This sink may be the onshore data center or a simple sensor node over the water surface. In data forwarding procedure, the source (Source: The words source node and initial node are alternatively used for the source) node generates data packets and communicate with its neighbors to find the potential node. Afterward, the potential neighbor node finds the next potential node from its neighbors and transmits data packets towards that potential node. To find the potential neighbor from the forwarder node, some criteria and routing procedures are defined. This criterion may base upon efficient energy use or alleviation of void holes. Radio Frequency (RF) waves cannot be used in an underwater medium. The reason behind is that RF technology increases the energy consumption by increasing the attenuation factor. At lower frequencies (30 to 300 HZ), water becomes a conductor for RF waves. Moreover, the frequencies in this range require the large size of antennas and these antennas require high transmitting power for data transmission. These requirements cannot be fulfilled in UWSNs. Consequently, RF waves cannot be used in UWSNs. Additionally, the technology of optical waves requires very high precision on a single point for a transmitter and a receiver. However, sensor nodes move with the water current. In essence, we have to use the acoustic waves in UWSNs. Nevertheless, the speed of acoustic waves is almost five orders of magnitude less than the speed of RF waves [2]. The underwater medium is extremely unpredictable and challenging when compared with Wireless Sensor Network (WSN). The major differences of UWSN with WSN are: (1) high energy consumption (2) high propagation delay (3) low bandwidth (4) high dynamic topology-operation (5) less propagation speed (6) low efficiency (7) low data transmission rate and (8) high environmental and noise interferences. The comparison between UWSN and WSN is shown in Table 1.  
The design of routing protocols has paramount importance in UWSN. These protocols indicate the routing path for data from the source node at the bottom towards the sinks node at the surface of the ocean. Expressly, these protocols face the different challenges which are associated with the underwater medium, e.g., limited battery resources, interference, noise, reliable Packet Delivery Ratio (PDR), high propagation delay, movements of sensors and void holes. Efficient energy usage is one of the most important tasks during the design of a routing protocol. As the batteries of sensor nodes in an underwater environment are non-removable and have limited energy storage. This issue provides a strong base for efficient battery use. Mostly, energy dissipates during the processes of data packet transmission and reception. Efficient energy usage depends on various factors. For instance, the initial position and number of anchor nodes; sensor nodes and the way in which nodes are deployed. The deployment of a network must be one of the two types (1) sparse deployment and (2) dense deployment.