Wireless Sensor Networks (WSNs) are often deployedrandomly;this makes the positiondetermination of deployed nodes a very difficult issue, which is called localization problem. The importance of node localization information becomes from the facility of routing operation and the network control, that makes the network works correctly. Nowadays, Global Position System (GPS) is appeared as the most important position system, but it is inapplicable for the low-cost self-configure sensor networks, and also it is impossible to install GPS for each sensor nodebecause of high cost, large volume and high complexity required of adding it to nodes, especially for large network.Therefore, a few number of nodes may be configured with GPS. In this paper,we will study the localization algorithmAd-hoc Positioning System (APS) algorithmused inUnderwater WSN, andwill testthe effectiveness of usingitin determining the node position based on the distance calculated by anchor nodes. These nodes are the nodes which know their positions.