## 2 Related Works  
The complexity and time variation of the underwater network environment have encouraged researchers to study efficient and reliable data transmission methods from different perspectives. The routing protocol is the key to realizing data transmission in underwater wireless sensor networks. According to the implementation method, the routing protocol is divided into the geographic routing protocol [33; 34] and the opportunistic routing protocol [7; 16]. The geographic routing protocol uses the relative or absolute location information of underwater nodes to design the packet-forwarding path, avoiding a large amount of broadcast information caused by the routing search [35]. The opportunistic routing protocol solves the problem of intermittent connection caused by mobility and the dynamic underwater acoustic link in sparse acoustic wireless networks, and uses'store-carry-forward' opportunistic routing to maximize packet forwarding. According to the guaranteed type of quality of services (QoS), routing protocols are divided into those that focus on energy consumption balance, transmission delay, network life, or other indicators. Routing protocols can also be divided into traditional routing protocols and intelligent routing protocols according to whether artificial intelligence algorithms are used [3; 36]. Traditional routing protocols do not need data learning and model optimization. In specific network scenarios, the optimization effect of specific indicators is good, but the network adaptability is poor, and there is only one optimization goal. The intelligent routing protocol can collect the network status data, form awareness of the network environment through data learning, support the dynamic adjustment of routing, and realize the balanced optimization of multiple QoS indicators, and has strong network adaptability [36; 37]. Researchers have also adopted a cross-layer design method to further improve the performance of the routing protocol through the joint design of data link layer, transport layer, and application layer protocols [10; 38]. Several typical data transmission algorithms and their characteristics are listed in Table 1. The communication efficiency mainly reflects the ability of the routing protocol to send effective information and control the transmission delay and energy consumption. RLOR uses opportunistic routing to improve the reliability of routing. The basis of RLOR's selection of relay nodes is the use of transmission direction and distance, channel quality, number of neighbors, and residual energy of relay nodes to ensure that the best route can be dynamically selected. RCAR takes more account of the congestion avoidance method of relay nodes. In RCAR, the transmission delay and energy consumption distribution in underwater data communication are optimized. MCR-UWSN divides the network into multiple clusters; dynamically selects cluster heads based on the optimized heuristic algorithm; and constructs inter-cluster routes to achieve efficient data transmission. KACO adopts clustering and heuristic algorithms for network layering and clustering. PB-ACR can distinguish the priority of different applications. In PB-ACR, high- priority applications choose the shortest route, and low-priority applications choose the route with more residual energy. ACOR supports the adaptive adjustment of data sampling frequency according to data correlation to reduce transmission energy consumption. Although both RLOR and RCAR can provide good QoS under extremely limited transmission conditions, it is not enough to simply consider overcoming the problems of the underwater environment and seek the best transmission route. Dynamic network traffic will seriously affect the performance of routing protocols. These protocols also need to be combined with applications to achieve efficient information transmission, rather than just data transmission.