## 1 Introduction  
Underwater wireless sensor networks (UWSNs) are a technological solution used today for multiple applications, such as ocean monitoring (seismic, pollution, oil spills), early warning detection (tsunamis), or surveillance operations (underwater target location, surveillance of a country's aquatic territory). Nowadays, the scope of UWSNs is growing in applications such as deep sea monitoring, i.e., the network SeaWeb (a UWSN implemented by means of digital signal processing telesonar acoustic modems to help in connecting fixed sensors with the mobile sensors), or in energy saving programs such as Project Natick from Microsoft (underwater datacenter cooled by the ocean to save electricity costs) [1]. However, the performance of UWSNs is affected by a wide range of problems and challenges: changes in network topology, low bandwidth, high ocean noise and interferences, and high propagation delays, among others. In addition, there are many limitations in these networks, such as low reliability, reduced network throughput, and inefficient energy consumption [2]. UWSNs were deployed initially based on the concept of terrestrial WSNs (TWSNs), but due to the differences cited, the technologies would become different. Moreover, TWSNs use radio frequency as a communication medium and employ traditional protocols such as Transmission Control Protocol (TCP), which is not suitable for underwater communications [3]. The multiple constraints which affect the performance of UWSNs should be considered in the design of the network. These restrictions include attenuation, noise, and delays due to changes in water temperature, pressure, salinity, oceanic currents, etc. UWSNs are composed of sensor devices used for sensing, processing, and communication. For efficient communication, in the initial UWSN setup, several different functions are performed: data gathering, data scheduling, forwarding the data, data aggregation, localization of the nodes, and data fusion. During these phases, the most important requirement is to save the battery energy because of the limited power capabilities of sensor nodes, which collect and forward the data to the sink node for processing [4]. Based on the challenges faced by UWSNs, it is obvious that they consume more energy than TWSNs. Thus, to address these issues, the use of routing protocols is effective [5]. Generally, these routing techniques are classified into two types: proactive and reactive. The proactive routing protocol is a table-driven protocol, and the forwarding delay is short because the node already knows where to forward the packet consulting the table stored. All nodes store the route information and change it accordingly as the network changes. On the contrary, the reactive routing protocol finds the route on demand by sending many request packets, which causes large delays. UWSN routing can take advantage of opportunistic routing which is an emerging technique because of its remarkable capability to enhance network throughput, reliability, and energy consumption [6]. The performance of UWSNs can be improved by taking advantage of data reception at neighboring relay nodes and their cooperation in forwarding the data to the next hop. Since the sensor nodes have limited energy, it is impossible for the nodes to forward the packets by utilizing only one-hop routing; therefore, multihop routing is preferred. The problem is when source nodes send the data to all their neighbor nodes overhearing the packets within the transmission range, owing to the propagation nature of wireless communication. Thus, selecting an efficient forwarding relay is the main task [7; 8]. Besides many other challenges in UWSNs, the packet forwarding or relaying through a node with energy efficiency and low end-to-end (E2E) delay will be crucial, while the second best relay node will be selected among all the remaining candidate nodes [9].