For detecting underwater targets, a total-field scattered-field source FDTD is proposed. Two FDTD simulations are included. Tangential electromagnetic fields on exchange surface are recorded in first simulation with fine grids. In second simulation, equivalent source is added with coarse grids at total-field scattered-field surface. Ratio of coarse grid to fine grid can be extremely large, such as N=10 while run time and consumed memory are reduced. Precision and validity of the method in calculating radiated fields of line current source are verified. Finally, the method is utilized to analyze electromagnetic response of underwater space with obstacles. Flexibility of the method in simulating a realistic underwater electromagnetic problem is illustrated. It is pointed out that delay time and magnitude of timing waveforms at receive points are different with conductivity difference of abnormal bodies.