Tracking of underwater vessels by the means of passive sonars is one of the most challenging problems in surveillance purposes. In this paper, the tracking model of Hassab, which is used by F. El-Hawary and G. A. N. Mbamalu for developing underwater tracking algorithms, is examined, with the presumption that the modeling cannot be robust, because these algorithms do not consider the sound speed variations, which vary according to the depth, temperature and salinity of underwater environment. In this paper, it is tried to introduce an improved tracking model for tracking the underwater targets, based on Hassab model, by considering the sound speed variations and then the models are compared together.