In this study, the multiphase poroelasticity theory is used to develop a theoretical model to analyze the horizontal and vertical motion at the surface of a gassy ocean sediment layer induced by obliquely P waves. The marine layer is lying on the elastic solid bedrock and modeled as porous material with finite thickness, which is saturated by two compressible and viscous fluids (liquid and gas). The analytical solutions for the wave reflection and transmission in the coupled seawater-sediment-substrate system are obtained. Then, the theoretical formulation is derived for the computation of motion amplitudes in both horizontal and vertical components. The displacement amplitudes, displacement ratios and amplification coefficients are examined numerically with incident angle, wave frequency, thickness of porous layer and saturation degree of sediment. It is demonstrated that even a slight change of gas content in the porous sediment may lead to significant influence on seismic dynamics of porous sediment layer. In addition, apart from saturation degree, this influence also depends on the incident angle, wave frequency and thickness of sediment layer. The study shows that saturation condition need to be carefully taken into account for the interpretation of field observations on seismic ground motions of the seabed.