## 2 Related Work and Motivation  
In recent years, a lot of research has been carried out on routing protocols of UWSNs. [PERSON] _et al._[8] suggest an energy-efficient routing protocol which jointly utilizes single-hop and multi-hop communication; however, it also allows high delay in data transfer. [PERSON] _et al._[9] suggest  
Figure 1: Cylindrical network. another application-oriented routing protocol for UASNs. They propose an energy-harvesting technique specifically for the underwater moored system. They also formulate the model for energy consumption and path loss in underwater environemnt. Hop-by-Hop Vector-Based Forwarding for underwater sensor networks (HHVBF) [10] is another localization based protocol which assumes vectors formation between the transmitting and receiving nodes. In this way, it reduces control overhead and end-to-end delay of the network. In [11], authors analyze the path loss and the effects of wave movement on acoustic signal. They also provide improved computational model for path loss of a signal. Collection of data through chain formation is an efficient way in terrestrial Wireless Sensor Networks (WSNs). It saves the energy of sensor nodes and minimizes delay. In rectangular and cylindrical network, chain formation offers improved performance by finding both local optimum and global optimum solutions for data gathering. There are many routing protocols in WSNs which work on the principle of chain formation. PEGASIS [12] is one of the chain based protocols. In this scheme, greedy algorithm is implemented. Initially, chain leader node if found out which is the farthest node from BS. Each node then communicates with its closet neighbor. Soon after chain establishment, a node with highest residual energy is selected as chain leader node. The chain leader node is responsible to directly communicate with BS as it gathers data from all other chain member nodes. Thus, the energy consumption cost is minimized to some extent. However, the length of over all routing path is somewhat increased which leads to increased energy consumption. There are many improved versions of PEGASIS. In [13], authors propose an improved multi-edge chain to minimize distant neighbor problem in which a long link is created in the chain. Similarly, [14] considers distance, energy and congestion while constructing chain. As a result, these schemes improve the network lifetime in an efficient manner. Multi-Path Transmission (MPT) proposed in [15], minimizes the challenges at the physical layer by proposing source-initiated and power-controlled transmission. It also provides reactive routing for on-demand data applications. Another reactive routing protocol, improved Adaptive Mobility of Courier nodes in Threshold-optimized Depth-based-routing (iAMCTD) [16] improves the network throughput and largely minimizes packet drop ratio by using its formulated Forwarding Functions _FFs_. To reduce network lifetime, mobile courier nodes are utilized in UWSNs. In [17], authors use autonomous underwater vehicles to minimize end-to-end delay and is specifically designed for delay-sensitive applications for UWSNs. In UWSNs, sensor nodes also transfer the data towards the underwater vehicles. They use both direct transmission and multi-hop transmission. In another application, nodes sense the moving targets through UASNs. [PERSON] _et al._[18] suggest a scheme to track moving targets on the basis of particle filter technique. They also combine filter technique and interacting-model method. Moreover, there are some energy-efficient data routing schemes such as Link-State Based routing (LSB) [19] and Round-Based Clustering (RBC) [20]. These protocols minimize the major problems in acoustic routing such as energy dissipation of nodes, high end-to-end delay and high path loss by using different methods. Some of these techniques, tackle the problems more realistically than the other schemes by considering the node's mobility and shallow water conditions.