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This project began with an effort to understand and utilize environmental and deliberately produced electromagnetic (EM) signals in the coastal ocean. Our studies were undertaken to provide a means to communicate with and guide autonomous underwater vehicles (AUV) and other autonomous sensor systems, such as moored instruments. There is a need for non-acoustical methods to navigate and control AUVs, communicate with autonomous instruments, and detect submerged and buried objects in shallow water. In principle, extremely low-frequency EM signals ( 1 kHz) can be used for these purposes. In practice, however, not enough is known to predict reliable signal-to-noise ratios for particular applications. This is both because the propagation paths in the coastal environment are complex and because the background environmental fields are not well understood. Clearly, a comprehensive numerical model is needed to determine the effects of various complications encountered in the littoral region. We undertook an effort to develop a numerical model that includes many EM influences, such as environmental noise, seabed electrical conductivity, variable bottom depth and coastline, and deliberate EM signals. Our goal is to understand these factors well enough to reliably estimate signal-to-noise ratios in planning coastal EM operations and to improve basic understanding of EM propagation and environmental noise in the coastal waters and to make and interpret sensitive EM measurements. The project had two components: 1) theoretical and numerical studies and 2) instrument construction and field observations.