Marine underwater electromagnetic field plays an important role in the prediction of the marine disasters, such as earthquake and tsunami. A detection method for the weak marine electromagnetic anomalous signatures generated by the earthquake and tsunami is presented. In order to implement the anomalous signal detection, the ocean current meter and tidal wave meter are integrated into the marine environmental underwater electromagnetic field measurement system, and the electromagnetic field data and environmental data such as sea current and wave height can be measured synchronously. The measured environmental data should be used to construct the prediction model of marine underwater electromagnetic field induced by the sea motion. Based on the prediction model, the theoretical values of electromagnetic field induced by the sea motion are estimated. This real signature induced by the sea motion can be extracted from the measured underwater electromagnetic data by use of the coherent filter method, and the remaining anomalous signature can be used to analyze the earthquake and tsunami phenomenon.