Abstract : Fighting aigainst the corrosion of underwater structures and ships need the use of a well-designed cathodic protection. This protection makes currents flowing in the seawater and so an electromagnetic field, detectable by sensors. The first idea of the work is, from the knowledge of the geometry and the physical behavior of the structure, to predict the electromagnetic field. The second idea is to develop an inverse method of corrosion diagnosis from electromagnetic measurements in the seawater, to find the damaged areas. Thus, from a set of measurements in the seawater, a corrosion diagnosis is possible and an extrapolation of the electromagnetic signature at further depths is available. This method is finally experimentally checked on simple cases and with a complex ship mock-up.