## 1 Introduction  
To explore undersea available resources, UWSN is most feasible and effective solution. The aforesaid paradigm offers unique applications, for instance, inhabitant monitoring, tactical surveillance, disaster prevention, resource investigation, etc. [1; 2; 3]. UWSNs are different from the terrestrial ones in many aspects: (i) instead of radio signals, UWSNs use acoustic signals for communication; (ii) the topology of UWSNs is more dynamic than the terrestrial sensor networks; (iii) the deployment in underwater is relatively sparse; (iv) node localization in UWSNs is difficult as compared to terrestrial sensor networks [4; 5; 6; 7]; and (v) the sensor nodes are energy limited and it is infeasible to replace or recharge their batteries after deployment [8]. The UWSNs face many challenges such as low bandwidth (\(<\)100 kHz), high propagation delay (1500 m/s) and high bit error rate.  
Depth based routing [9; 10; 11; 12; 13] shows high packet drop due to void hole problem. Weighting depth and forwarding area division DBR (WDFAD-DBR) [14] only considers depth difference between two hops to avoid void hole. However, considering two hops does not eliminate void hole. The void hole is an area from where data packets cannot be delivered to the destination. This is because of the unavailability of nodes in the communication range of the source node. It could occur at the time of random deployment when nodes get placed far away from each other and are never able to communicate with the network nodes. Moreover, it could be due to displacement of nodes with water currents as well as because of high burden of data which suddenly depletes node battery.  
To resolve the issue of void hole problem, various routing strategies are presented by the research community, e.g. WDFAD-DBR [14]. This algorithm assigns weights to neighbor nodes for effective forwarder selection and avoids immutable nomination of the forwarder node. Although it assigns weights to all neighbors and computes holding time to save battery, it considers only one selection parameter which is distance and obtains neighbor information up to two hops. This scheme gives more weight to distance parameter, which enables selection of the same forwarder until its death. Thus, the void hole is not avoided at all.  
There is also a backward transmission mechanism which finds alternate paths to deliver data at the destination [15]. It traverse backward nodes, until a neighbor is available which can deliver the data successfully to the base station. However, there is a problem of communication overhead which degrades the network performance. Thus, a protocol is desired which can minimize the communication overhead to increase network lifetime. In this regard, a protocol which uses unique hop identifiers to minimize overhead and improves the network lifetime along with the void avoidance [16]. However, this scheme is not effective because in harsh acoustic environment, water currents are major influential factor which make it difficult to obtain hop id all the time.  
Therefore, to avoid void hole, new routing protocol(s) needs to be proposed. In proposed routing protocols, we use sender based approach to avoid void hole and reduce the probability of collision by considering channel inference. Moreover, the proposed protocols also reduce duplicate packet transmission. It is important to note that this research work is extended form of the work in [17].  
The rest of the paper is organized as follows. In Section 2, we discuss some well-known routing protocol with their drawbacks for UWSNs. Details of the proposed schemes are given in Section 3. Section 4 presents theoretical analysis. A detailed analysis about the performance evaluation of proposed protocols is given in Section 5. Finally, conclusions are drawn in Section 6.