The purpose of the thesis was to implement the Finite-Difference Time-Domain (FDTD) method for low frequency electromagnetic wave propagation in seawater. One specific problem has been the truncation of the computational domain at the air/sea surface. For this purpose I have developed and tested an impedance boundary condition (IBC), which simulates the influence of air space. By the use of this IBC you only need numerical values from the water volume. For validation of my code I have used NLAYER, a solver for electromagnetic wave propagation for horizontally stratified media developed at FOI. I found that the IBC at the air/water boundary gave good results if the computational box was big enough. Another condition was that the receivers cannot be placed too close to the source.