<p>Background Seismic analysis in marine geotechnical engineering is computationally intensive. Conventional methods for accounting for the influence of infinite water domains are often costly and time-consuming.</p><p>Objective To propose a simplified anhydrous substructure model that significantly reduces computational cost while maintaining accuracy for efficient seismic analysis of ocean engineering sites.</p><p>Methods The model employs the finite element method to discretize the seawater, saturated seabed, and bedrock as an integrated system. Its core innovations involve the precise derivation of the coupling interface condition between the water and the saturated medium, and the introduction of an exact artificial boundary to model the radiation damping effect of the infinite water domain. The seismic input, based on one-dimensional free-field response, is extended to the artificial boundaries of each medium layer via Snell's law to obtain equivalent seismic loads. Following this, and assuming water incompressibility, the expression for dynamic water pressure on the saturated medium surface is derived and incorporated into the global finite element equation as an added mass model.</p><p>Results The proposed anhydrous substructure model achieves a significant improvement in computational efficiency while ensuring analytical accuracy.</p><p>Conclusion The model's correctness and its applicability to sites with structures are verified through comparison with a full-domain model. It provides an efficient and reliable numerical tool for seismic analysis in ocean engineering that considers seawater effects, demonstrating considerable potential for future engineering applications.</p>