To estimate microwave emissivity of sea foam consisting of dense seawater-coated air bubbles, the effective medium approximation is applied by regarding the foam layer as an isotropic dielectric medium. The Rayleigh method is developed to calculate effective permittivity of the sea foam layer at different microwave frequencies, air volume fraction, and seawater coating thickness. With a periodic lattice model of coated bubbles and multilayered structures of effective foam media, microwave emissivities of sea foam layers with different effective permittivities obtained by the Rayleigh method are calculated. Good agreement is obtained by comparing model results with experimental data at 1.4, 10.8, and 36.5 GHz. Furthermore, sea foam microwave emissivities calculated by well-known effective permittivity formulas are investigated, such as the Silberstein, refractive model, and Maxwell-Garnett formulas. Their results are compared with those of our model. It is shown that the Rayleigh method gives more reasonable results.