Applications of Underwater Sensors Networks (USN) include environmental ocean/sea monitoring, underwater mine searching, detection of chemically/biologically harmful substances or pollutants, autonomous underwater attack/defense systems, collecting ambient data for ship navigation, etc. The success of terrestrial sensor networks has promising results for more challenging environments, such as underwater. Using groups of sensors communicating with each other yields better performance in underwater applications rather than single sensor equipments. However, forming a network it? underwater is not straightforward. The main challenges arise from the physical communication medium and affect the zipper protocol stack such as MAC, routing, transport layer protocols and localization. For USNs, protocols at all those layers should be designed with communication and energy cost in mind. Localization is required for data tagging. In terrestrial sensor networks, either GPS is used wherever available or several GPS-free, messaging-intensive schemes have been used However, both are unsuitable for USNs. In this paper, we compare the performance of two distributed localization methods tailored for large-scale USNs.