## 2 Literature Review  
In this section, the existing protocols are discussed, which are energy efficient and achieve data reliability with maximum throughput. The state-of-the-art depth-based routing protocols are categorized into two types: multi-hopping via single path and multi-hoping via multiple paths. Single path concept is used for minimum EC to avoid void hole regions. In contrast, multi-path routing helps to ensure reliable data delivery towards the destination. Pictorial form of the aforementioned literature is presented in Figure 1.  
The author proposed the Geographic and Opportunistic Routing with Depth Adjustment (GEDAR) in UWSNs in [9]. This protocol performed anycast geographic opportunistic routing. Therefore, the selection of next potential neighbors is anycast. Recovery mode is active in proposed protocol to manage the void hole region and to move the nodes into a new location after depth adjustment recovery method. GEDAR efficiently solved the problem of void hole region. It also increased the PDR through depth adjustment mechanism. However, position adjustment mechanism consumed much energy and increased end-to-end delay in the data packet delivery. The routing protocol HYDROCAST is proposed in [10] to route the data packets towards destination by measuring pressure level information by [PERSON] et al. Anycast mechanism can be followed to route the data packets in only vertical direction. Data from a single source is forwarded to a subset of forwarders using a greedy approach. The receiver does not forward the data packets until all forwarder nodes fail. Data is transferred to the higher depth nodes when there exists a possibility of the data to enter in void hole regions. In this case, data is directed towards sink using different paths. Protocol's reliability is enhanced by increasing transferring DR towards forwarders through different paths. This mechanism helped to avoid void hole regions in the network. However, HYDROCAST protocol must bear the cost of both energy and end-to-end delay simultaneously. Another Layered Multi-path Routing Control (LMPC) protocol is proposed in [11] by [PERSON] et al. LMPC sends many copies of data packets towards a destination through different routes. To achieve the reliability in data binary tree generation concept is introduced in the proposed protocol. Generation of a binary tree from the source node to destination enhanced the EC. However, sparse area generation with randomly deployed nodes result in packet drop and the unsensed zone is created in the network. [PERSON] et al. proposed two routing protocols in [12] which are depth-based and known as: FLMPC-One and FLMPC-Two. Both proposed protocols followed the positive aspect of LMPC. The features which differentiate FLMPC from LMPC is that FLMPC generates a binary tree from sensor node instead of the source node and cover the void hole regions problem by introducing cross nodes  
Figure 1: Pictorial Description of Depth-Based Protocols. concept. The proposed protocol reduced the number of retransmissions. However, 3-hop neighbors information and generation of binary tree results in high EC. In [13], [PERSON] et al. proposed a protocol which depends on weighting depth and forwarding area division (WDFAD-DBR) for UWSNs. It is done to increase the reliability of data by decreasing the probability of data packets to enter in void hole regions. The authors measured the sum of depth differences from the source node to next expected hop node to avoid void hole regions. Theoretical analysis is performed in this article using channel contention. Energy efficacy and data reliability are successfully achieved from the proposed protocol. However, sparse regions effect Packet Delivery Ratio (PDR) with the propagation and processing time at every hop node. In [14], Geographic Forwarding based on Geo-spatial and Greedy Forwarding Division (GFGD) and Greedy Geographic Forwarding based on Geo-spatial Division (GGFGD) protocols are proposed by [PERSON] et al.