Using numerical and analytical modeling, we assess the feasibility of marine controlled-source electromagnetic (CSEM) data for monitoring the flooding front during water flooding of an oil reservoir. We discuss the ability of time-lapse CSEM data to resolve changes in the electric conductivity small enough to be of interest for monitoring purposes. Measurement and modeling errors are discussed briefly, and analytical calculations comparing time-lapse signals with a certain type of time-lapse modeling errors are performed. Numerical calculations, performed with a volume integral-equation method, are used to study the effect on the electromagnetic (EM) fields of relevant conductivity changes. The numerical investigation includes robustness with respect to survey parameter values, discrimination between different front shapes, and ability to overcome time-lapse error. Simulated signals are found to be strong enough to overtake typical measurement errors and are fairly robust toward perturbations of survey parameters. It is found analytically and numerically that certain modeling errors experience a high degree of time-lapse cancellation.