Underwater hydroacoustic measurements are widely used in sonar applications not only for the detection of target objects (such as vessels or fisheries) but also to characterize the nature of the seabed and monitor the coastal environment. In this present work, hydroacoustic measurements performed by a single beam narrowband echosounder are modelled using a Sommefeld integral representation. This experimental setting allows the insonification of a specific location of the seabed, and the reflected signal received by the acoustic device is used to quantify the properties of the seabed. Typically, the vibroacoustic behaviour of the coastal seabed is modelled by using homogeneous rigid-framed porous or poroelastic media. However, since granular materials with different pore sizes are also commonly found in coastal environments, consolidated and non-consolidated porous granular models have been considered, taking into account parameters such as porosity, tortuosity, flow resistivity and standard deviation of the pore size. Since the underwater fluid domain has heterogeneous properties (strongly dependent on temperature and salinity in coastal environments), a displacement-based finite element method combined with a Perfectly Matched Layer technique has been used to compute the time-harmonic solutions of the scattering coupled problem involving the seawater and the granular porous seabed domain. The quantification of the biological presence in those underwater environments, such as algae on the surface of the coastal seabed or the bivalve concentration buried in the upper layer of the sediment, will be studied by analyzing their effects on the reflected signal recorded on the free surface of the sea, where the echosounder is placed.