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The field of Underwater Sensor Networks attempts to set up a Wireless Sensor Network (WSN) underwater for the purpose of exploration and monitoring by addressing challenges unique to the oceanic environment. In an UnderWater Sensor Network (UWSN), the knowledge of the location of a node can both improve the operation of the network (e.g. geographical routing) and add significance to the data that is collected. Many localization techniques used in UWSN are based on multilateration. Multilateration assumes the underwater speed of sound to be constant. However, several studies have shown that this speed varies with salinity, temperature and pressure. This variation incorporates an error into the results obtained from localisation. This paper presents an algorithm to find an estimate of the underwater speed of sound at a particular location and time. This estimate can be provided to multilateration to improve the localization accuracy. The existing multilateration technique need not be modified. Simulation studies show that our algorithm improves the localization accuracy over a pure multilateration technique that assumes the underwater speed of sound to be 1500 m/s.