Abstract: Systems and methods for measuring water properties during a marine survey are disclosed. While electromagnetic-field (“EM-field”) receivers located along streamers towed by a survey vessel measure surrounding EM fields, horizontal conductivity profiles of a body of water located above a subterranean formation are also measured. By inputting a substantially continuous, conductivity profile of the body of water along with EM-field data into an EM inversion process, estimates of the subterranean formation properties, such as resistivities, can be generated with a higher degree of confidence than estimates of properties based on a speculated or sparse conductivities of the body of water.