In this paper, we analyze the problem of localizing and tracking a mobile node in an underwater environment with an isogradient sound speed profile (SSP). We will show that range-based localization algorithms are not so accurate in such an environment, and they should be replaced by time-based ones. Therefore, we relate the mobile node location to the travel time of a propagating sound wave from (to) an anchor node to (from) the mobile node. After obtaining sufficient time measurements, positioning can be achieved through multilateration. To accomplish this, we utilize the extended Kalman filter (EKF) for multilateration and tracking the mobile node's location in a recursive manner. Through several simulations, we will show that the proposed EKF algorithm performs superb in comparison with algorithms which assume a straight-line wave propagation in an underwater environment.