Ocean resources are crucial for the sustainable development of a country, and improving marine resource surveys has become a focal point for many nations. Accurate detection of underwater targets has become a hot topic in the field of geophysics. The electromagnetic exploration method, based on differences in resistivity, can effectively extract and identify underwater high-resistivity or high-conductivity targets, making it one of the key technologies in marine geophysical exploration. Airborne electromagnetic methods, using airborne platforms, enable rapid data acquisition in marine areas and have been widely applied in marine resource surveys and environmental monitoring. However, the marine environment is complex and dynamic, and the forward modeling and inversion interpretation techniques based on one-dimensional marine-land geoelectric models struggle to characterize three-dimensional complex geoelectric structures. This severely hinders the detailed interpretation of multi-frequency electromagnetic data from airborne platforms. In this study, a hexahedral mesh spectral element method is used for high-precision multi-frequency airborne electromagnetic three-dimensional forward modeling of the marine-land geoelectric model. Numerical simulations and analysis of frequency-domain electromagnetic detection of underwater targets are conducted, summarizing the propagation characteristics of frequency-domain electromagnetic fields under different conditions. The study explores the identification capability of multi-frequency electromagnetic systems for underwater targets, providing theoretical references for the effective detection of high-conductivity targets in low-altitude marine areas using multi-frequency electromagnetic systems.