Theoretical results regarding surface waves in the ocean are most often obtained assuming that the flow is irrotational in order to greatly simplify the problem's formulation. This is also a key hypothesis when deriving reduced models like the (non-linear) Shallow Water, the Green-Naghdi or the KdV equations. In the first part of this talk, we shall propose a reformulation of Euler's free-surface system describing breaking waves, assuming that the vorticity vanishes identically in order to reduce the problem's intrinsic dimension. Secondly, we will discuss the validity of this important hypothesis investigating the stability of boundary layers in near-shore oceanic flows using DNS of the full Navier-Stokes system.