## 2 Related Work  
The main function of the route protocol is to decide on the best route for a packet. It forwards data according to the selected route and provides network connectivity. It is the basis for nodes to communicate with each other. Many different routing protocols have been presented and used for the UASNs due to the unique characteristics of underwater acoustic channels. In this section, we mainly review the related work of routing protocols for UWSNs. According to whether the routing table is maintained, self-organizing routing protocols can be divided into two categories: (1) opportunistic routing protocol; and (2) deterministic routing protocol. Opportunistic routing protocol is a one-packet-one-route protocol, which takes advantage of the broadcast characteristics of wireless underwater communication and selects a group of suitable neighbors as potential forwarders. Deterministic routing protocols establish one or more reliable routing paths before data are sent and transmit data hop by hop along the paths until the data are transmitted to the destination [14]. Vector-based forwarding (VBF) [15] protocol is an early typical opportunistic routing protocol. By using the location information of nodes to limit the scale of flooding, it can save energy consumption and handle the mobility of nodes. In recent years, there are a lot of research results on opportunistic routing. Adaptive-location-based routing protocol (ALRP) adopts strategies of forwarding area, delay and probability to increase the forwarding efficiency [16]. Distance vector-based opportunistic routing (DVOR) uses the query mechanism to establish the distance vector toward the destination nodes for each node, and then forwards the packets to the destination along the opportunistic shortest path [17]. It can be seen that they do not uphold routing tables, resulting in every packet necessitating a routing discovery process. The selection of intermediate forwarding nodes is based on either latency or probability, which will seriously affect the performance of the protocol. Therefore, opportunistic routing is easy to cause packet transmission delay, packet loss and other problems. This kind of routing protocol is more suitable for the network scenario of all mobile nodes. Ad Hoc On-demand Distance Vector, the most popular one among deterministic routing protocols, uses the on-demand route discovery as in a temporally-ordered routing algorithm (TORA), has the maintenance characteristic of dynamic source routing (DSR), and employsthem in a hop-by-hop routing scheme instead of source routing. The mechanism of dynamic discovery and maintenance of routing is more suitable for underwater wireless sensor networks with static nodes and mobile nodes. AODV uses a flooding route discovery mechanism and periodically sends hello packets to maintain routes. When directly applied to underwater acoustic networks, it will lead to excessive redundant retransmissions, packet collisions and unnecessary energy consumption. This phenomenon is called broadcast storm. Figure 2 shows the route discovery process of the AODV protocol. AODV does not use auxiliary information, so the flooding mechanism is used during route discovery. On account of AODV using the flooding mechanism in the route discovery process, node E also participates in this process. However, there is no chance for node E to establish a route to the destination. The route discovery and maintenance mechanism of AODV needs to be improved. With the development of underwater acoustic positioning technology (LBL/SBL/USBL) and integrated navigation technology, the location information of underwater nodes can be obtained realistically, which makes it possible to introduce location information into underwater wireless sensor networks [18; 19]. And for many applications of UWSNs, it is meaningful to associate the data observed with the location information [20]. The reference [21] proposed a directed search AODV (DSAODV) protocol for underwater acoustic communication.