The sub-relativistic laser beam interaction with an underdense plasma is investigated via two-dimensional (2D) numerical simulations with respect to the laser polarization direction. Different parametric instabilities dominate the interaction depending on the propagation direction of the daughter waves in the simulation plane with respect to the laser field polarization and laser propagation direction. In the plane containing the laser electric field (p-polarization) the interaction is dominated by the two-plasmon decay instability and the beating of large amplitude electron plasma waves produces periodic ion density perturbations suppressing stimulated Raman scattering in the quarter critical density zone. A stronger absorption and heating of hot electrons is observed in the case where laser polarization is perpendicular to the simulation plane (s-polarization). Furthermore, by comparing a plane laser wave with a narrow beamlet, the effect of the initial transverse laser profile is proven to play an important role in exciting the filamentation instability, which competes with stimulated Brillouin scattering and affects the laser absorption and hot electron generation. A dedicated three-dimensional simulation indicates that a 2D simulation with p-polarization produces a more reliable results while the case of s-polarization overestimates the laser absorption and hot electron generation.