## 1 Introduction  
A group of interconnected sensor nodes through acoustic channel form a underwater wireless sensor network (UWSN). The collaborative behaviour of sensing devices in the network enables: monitoring of remote locations, physical environment, temperature, humidity, battlefield, oceans, volcanoes and many more [1, 2], whereas sensors are the key component of UWSN, which are randomly deployed over the specified network volume, to monitor, sense, gather and transmit the information of interest. In UWSN, sensor nodes have limited battery, which is key consideration while designing a routing strategy. Also the sustainable deployment of sensor system is required to reduce the deployment and operational cost to prolong the network operational time [3, 4]. In order to ensure successful communication among the nodes in acoustic network, the necessary factors are required to be considered in the design of a routing algorithm. For instance, the major factors associated with underwater channel need to be analyzed e.g., high delay of acoustic signal propagation because sound can propagate in acoustic environment with speed of 1500 m/s [4], high bit error rate because of noise and dynamic nature of acoustic medium, limited bandwidth, multi-path fading, etc. [5]. Therefore, an efficient routing strategy for acoustic channel is desired which balances energy dissipation to optimize the network lifespan [6]. For minimal energy consumption, geographic routing is widely accepted because of its scalable and simple implementation methodology [6; 7]. Moreover, the stateless nature of geographic routing allows it to communicate without establishing entire path from source to destination. This algorithm only computes one eligible neighbor which acts as a potential forwarder to relay the data packet. Additionally, this routing mechanism is highly effective when node density is high because it follows greedy forwarding mechanism to transmit the data in multi-hop manner [5]. While in sparse deployment, due to the greedy approach, nodes select an optimal route in terms of distance which results in immutable selection of the same node resulting in sudden depletion of the node battery [8]. This death of the node creates energy hole which results in the breakage of the data route because of which downstream nodes cannot deliver their sensed information to the base station. The aforementioned limitation is avoided through opportunistic routing (OR) paradigm, in which the selection of the forwarder set enures successful data delivery towards the destination node even if one node from the set fails, still, the data is delivered [9]. However, the delivery of redundant packets at base station degrade the performance of OR. To avoid the transmission of duplicate packets, control message exchange or holding time mechanism is used in opportunistic routing strategy. In the former approach, node with minimum distance and shorter route from the destination compared to nominated neighbors of the sender, is elected to deliver the data by acknowledging with control message that data is delivered successfully. In the later one, holding time is computed for each neighbor node to assign the priority in order to communicate on the acoustic channel. Incase of high priority node failure, node with second high priority in the set, transmits the data packet after its holding time expires. Still, in receiver based communication, duplicate packets from the hidden terminal regions are not suppressed. The hidden terminal is a region, where nodes lie in the transmission range of source node, but these nodes are unable to receive transmission or failure acknowledgement from the high priority node and ultimately transmit data packet towards the destination. Due to duplicate transmissions from the hidden terminal volume, unnecessary energy dissipates resulting in short network lifespan.