## 1 Introduction  
The majority of the earth's surface is covered by water. Various underwater applications have been researched, e.g., environmental monitoring, undersea explorations, disaster prevention, mine reconnaissance, _etc._[1]. Traditional monitoring systems are expensive and complicated. These equipments utilize individual and disconnected equipments to collect data from their surrounding environments [2,3]. The emergence of UWSNs provides new opportunities to explore the ocean. In UWSNs, conventional large, expensive, individual ocean monitoring equipments are replaced by relatively small and less expensive underwater sensor nodes that are able to communicate with each other via acoustic signals. Many technologies for UWSNs have been researched, e.g., medium access control (MAC) and secure routing protocols, localization technologies and time synchronization schemes. Localization is one of the most important technologies since it plays a critical role in many applications. Generally, there are three kinds of sensor nodes in UWSNs: anchor nodes, unknown nodes and reference nodes. Unknown nodes are responsible for sensing environment data. Anchor nodes are responsible for localizing unknown nodes. They can acquire their position in advance using GPS systems or artificial arrangement. Reference nodes consist of localized unknown nodes and initial anchor nodes. Localization process of an unknown node can be described as how the node determines its position by limited communication with several anchor nodes or reference nodes using some specific localization technologies. Although various localization algorithms have been proposed for terrestrial WSNs, they are not suitable for UWSNs [4,5]. The major difference between UWSNs and terrestrial WSNs is the different communication signals. Radio signal propagates at long distances through sea water only at extra low frequencies between 30 Hz and 300 Hz. Low-frequency radio signal requires long antennae and high transmission power. Relatively, acoustic signal attenuates less and travels further. Thus, acoustic signal is more suitable for UWSNs [6]. Acoustic communication channel has its unique characteristics. Hence, the existing localization algorithms for terrestrial WSNs cannot be applied to UWSNs. Currently, many localization algorithms have been proposed for UWSNs. Researchers in [2,3] classify these localization algorithms into two categories: distributed and centralized localization algorithms, based on where the location of an unknown node is determined. In distributed localization algorithms, each underwater unknown node collects localization information and then runs a location estimation algorithm individually. In centralized localization algorithms, the location of each unknown node is estimated by a base station or a sink node. These two categories are further divided into subcategories of estimation-based and prediction-based algorithms. Estimation-based algorithms use current information to compute the location of a node, while prediction-based algorithms aim at predicting the location of a node at the next time instant, using previous and current location information. Moreover, another early survey paper [7] researched several terrestrial localization algorithms and analyzed their suitability for UWSNs. However, localization algorithms for UWSNs have not been discussed. In UWSNs, the movement of sensor nodes caused by water currents is inevitable. The classification in [2,3] is not distinct enough without considering the mobility of sensor nodes. Thus, in this paper, we reclassify the localization algorithms based on the mobility of sensor nodes. The rest of paper is organized as follows. In Section 2, we focus on recent localization algorithms from 2006 to 2011 andreclassify the algorithms with a new perspective.