Localization issue is a crucial part of Wireless Sensor Networks (WSNs) including Underwater WSNs (UWSNs). Unlike in Terrestrial WSNs where the localization techniques are well established, localization for UWSNs is still at the infancy stage. Most of the existing localizations proposed are based on Time of Arrival (ToA) and many of them assume an ideal environment and precise synchronization among sensor nodes. This research has proposed a new localization technique for UWSN based on Received Signal Strength Indication (RSSI). Extensive study has been carried out on the application of Lambert W function for an accurate distance measurement within five iterations. The technique that utilizes Lambert W function and RSS has been developed. The technique is divided into three separate steps including initialization, distance measurement and position estimation. This new localization technique has a two-level computation approach that allows sensor nodes to have coarse estimations of their locations, while the sink, which has more resources, calculates accurate positions. The new RSS-based localization is compared to ToA-based localization using MATLAB with variety of oceanographic properties considered. The simulation results showed that the new localization technique can achieve far better accuracy in all conditions. Besides, the proposed technique is less susceptible to errors caused by the environment factors as compared to ToA-based methods. It is also power-efficient, as the main part of the localization computations is computed at the sink rather than sensor nodes.