## 1 Introduction  
As the most effective underwater communication method, underwater acoustic communication technology and underwater acoustic modems have been studied extensively and deeply [1]. With the development of ocean exploitation, point-to-point underwater acoustic communication cannot meet certain underwater application scenarios. In order to realize, explore and analyze the ocean, underwater wireless sensor network (UWSNs) technology is essential. UWSNs has emerged as a powerful technique for aquatic applications, providing autonomous guarantees for various activities like environmental monitoring, intelligence collection, assisted navigation and so on [2, 3, 4]. The underwater wireless sensors networks have developed from a single set of underwater static nodes to a 3-D hybrid model composed of static sensor nodes, human occupied vehicles (HOV), unmanned underwater vehicles (UUV) or surface nodes, as shown in Figure 1. Mobile nodes can move freely in the observation area and carry out observation tasks in key areas or undertake network data collection and other works. The failure of static nodes due to power, fault and the large-scale movement of mobile nodes will lead to a change in topology [5; 6]. Therefore, the network needs to have the ability of self-organization. Underwater wireless sensor networks provide a complete architecture that ensures reliable and efficient acoustic communication among underwater modems, thus making these applications viable [7]. Ref. [8] identifies a number of sources of acoustic signal loss: spreading and absorption loss (a function of distance and frequency); multipath reflections from the surface, obstacles, the bottom and temperature variations in the water; noise due to artificial and natural sources; and scattering from reflections of a potentially rough ocean surface. Many of these forms of loss are unique to acoustic communication at longer distances. [PERSON] et al. studied the sensitivity of sound speed fluctuation on the arrival delay of middle range in deep water. When receiver range increases from 20 km to 30 km, sensitivities of ray arrivals gradually increase [9]. Inspired by the benefits of short-range RF communication in sensor networks, UWSNs are generally a multi-hop transmission structure. Multi-hop networks can cover relatively larger areas since the range of the network is determined by the number of nodes rather than the modem range. UWSNs are significantly different from terrestrial sensor networks in many aspects: low bandwidth, high latency, limited battery, variable topology, and 3-D space [10; 11]. Special attention should be given to these facts when designing routing protocol. So, most of the traditional routing protocols for terrestrial wireless sensor networks are not suitable for UWSNs directly. In underwater wireless sensor networks, energy constraint is a crucial factor since sensor nodes usually run on battery, the nodes working in the underwater environment are generally equipped with battery power, and it is impossible or difficult to recharge them in most application scenarios. Energy issues directly affect the life of the entire sensor network [12]. The self-organizing routing protocol monitors the changes of the network topology, and discovers, selects, and maintains the route dynamically. This issue significantly impacts the overall network performance and stands as a central challenge. In this paper, we focus on the application of Ad Hoc On-demand Distance Vector (AODV) routing protocol [13], which are typical self-organizing routing protocols, in UWSNs and propose a route-focus  
Figure 1: Architecture of underwater wireless sensor networks. AODV(RFAODV) routing protocol for UWSNs. RFAODV takes the energy and location in route discovery and maintenance into account. The main contributions of the RFAODV are as follows. * During route discovery, RFAODV proposes a novel forwarding area to determine whether nodes participate in forwarding or not.