Electromagnetic wave (EMW) absorbers with broadband attenuation and long-term stability are important for applications in marine environments. Dielectric ceramics excel in terms of thermal and chemical resistance but offer limited impedance matching, whereas magnetic materials provide strong absorption but degrade rapidly due to corrosion. Herein, we present an engineering approach for polymer-derived ceramics that utilizes ferric crosslinking to integrate both magnetic functionality and hierarchical structure within a single system. By reacting iron(III) acetylacetonate with Si–H groups in polyborosilazane, a uniformly distributed ferric polymer network is formed. Subsequent pyrolysis drives carbon nanotube growth and Fe_xSi_y phase formation, yielding a distinctive hierarchical “mushroom-like” structure composed of SiBCN matrices, carbon nanotube stems, and carbon-encapsulated Fe_xSi_y caps. This structure promotes EMW absorption via magnetodielectric synergy, rich interfaces, and multiple scattering, whereas carbon-encapsulated Fe_xSi_y in the SiBCN matrix provides corrosion resistance. The effective absorption bandwidth (EAB, defined as a reflection loss of less than −10 dB) of h-SiBCNFe reaches 8.16 GHz, while it also has a corrosion potential (E_corr) of 0.033 V and an ultralow corrosion current (I_corr) of 0.63 μA·cm⁻². These features highlight a new design strategy for developing advanced EMW absorbers tailored for marine applications.