When the ocean seabed is considered to be rigid, the ideal waveguide model can be used to model the shallow water environment. However, a more realistic ocean waveguide model treats the ocean floor as a boundary between two different fluid media. In this paper, a frequency-domain characterization of shallow water environments is proposed based on this realistic waveguide model with a fluid boundary. First, the time-frequency characteristics of this model are studied as well as the impact of the environment parameters on the dispersive phenomena. Then a frequency-domain matched filter receiver is designed to obtain time-dispersion diversity once we separate the modes using warping techniques in the time-frequency plane. Simulations demonstrate that the new receiver design improves the bit error rate performance.