## 1 Introduction  
As the amount of underground resources such as known stores of crude petroleum increases, the exhaustion of these resources is also being accelerated. Moreover, some countries that are rich in natural resources put pressure on other countries by limiting the export volume of their resources. Developing alternative types of resources is one solution, but the results so far have not been practical or cost-effective. The ocean, which occupies 70% of the surface of the Earth, has received attention related to resource development, not only for its size but also for its potential.  
Underwater Wireless Sensor Networks (UWSNs) are significantly different from Terrestrial Wireless Sensor Networks (TWSNs) in many aspects, such as their high latency, high error probability of each node, and their high communication cost [1]. UWSNs also have some disadvantages including the unavailability of real-time monitoring, limited interaction between an onshore control center and the monitoring instruments, and lack of a mechanism to detect failures quickly [2]. These characteristics make existing TWSNs unsuitable as UWSNs. Specifically, the major concerns in UWSNs are reduced energy use and reliable communication, since the sensor nodes are powered by batteries, which are difficult to replace or recharge in underwater environments and have a high error rate. Therefore, node clustering, which has been widely studied in UWSNs, is an effective technique to improve energy efficiency and to simplify the network architecture [3]. In addition, numerous studies have attempted to improve the reliability of the communication based on clustering protocols.  
A cluster-based UWSN is similar to a TWSN. The clustering protocols elect a head node, and other nodes transmit sensing data to their head nodes. The head node transmits the collected data to an underwater sink that is specially designed to communicate with the surface sink. In its role, the head node is more important in UWSNs than other nodes. If a cluster head failure is not accurately detected, it will unnecessarily execute a fault recovery process (head node reelection or network re-clustering) and thus, waste energy in the sensor network. To avoid this, it is important to be able to accurately detect head node failures [4].  
To reduce latency and improve reliability during the recovery process, we propose a checkpointing scheme, which stores the state of the head node and repairs the head node when it fails. The head node sends routing information and collected data to the backup nodes that periodically save this information. If a head node experiences a transient fault, one of the backup nodes detects the head node failure and becomes the new head node. The head node can quickly recover from a transient fault by omitting the reelection of a head node and by preventing the loss of the collected information. Even if a head node experiences a permanent fault, a backup node becomes the new head node immediately.  
The balance of this paper is organized as follows. Section 2 describes the architecture and background knowledge pertaining to underwater sensor networks. In Section 3, we describe some related works to improve the reliability of UWSNs. In Section 4, we propose the recovery scheme for a head node failure. In Section 5, we evaluate the proposed scheme. Finally, we provide some concluding remarks in Section 6.