Underwater acoustic sensor networking is the enabling technology for a wide range of applications including naval surveillance, oil platform monitoring, earthquake and tsunami forewarning, climate and ocean observation, and water pollution tracking. Underwater sensor nodes with sensing and communication capabilities form the underwater acoustic sensor network. Localizing the underwater sensor nodes is one of the fundamental tasks for UASNs where the location information can be used in data tagging, routing, and node tracking. Localization for UASNs is an active research topic where a large number of techniques have been proposed recently. This article provides an up-to-date survey of these techniques while pointing out the open issues.