A shear layer at the top of a submerged canopy was analyzed under oscillatory flow driven by free surface waves. By applying a dimensional analysis at the shear layer scale, a simple analytical model that reveals the dominant terms in Navier–Stokes equation was developed. A set of laboratory data shows good agreement with the analytical model and the assumptions applied in its derivation. The wave–seagrass interaction can be assumed as a horizontal parallel flow, given that the mean vertical velocity and the vertical advection component can be negligible compared with the mean horizontal velocity and the horizontal advection, respectively. Notable results are that the shear layer thickness (δ) can be approximated as a function of the horizontal orbital excursion (Aw) by δ≈0.2Aw. Additionally, the turbulent model parameterization as function of Aw shows good agreement in developing the shear layer, so that the mixing process at the top of the canopy and the development of the horizontal velocity profile are modulated by the horizontal orbital excursion and wave period.