AUV-based field measurement and characterization of the electromagnetic field in coastal waters associated with live submerged cables is described. The effort is in support of a program to characterize the EMF emissions associated with submerged power transmission cables from offshore energy harvesting devices to shore and identifying their environmental impact on marine animals. The electric field is measured using a custom 3-axes sensor mounted on a Bluefin 21 AUV and the magnetic field is measured using a Marine Magnetics SeaSPY magnetometer towed from the vehicle so that it is isolated from the AUV machinery. The AUV is equipped with onboard upward (600kHz) and downward (300kHz) looking ADCPs, and a CTD package which simultaneously provide the contextual in-situ oceanographic information, including temperature, salinity, and current velocity in the water column. The electric-field sensor development, its implementation on the AUV, and illustrative at-sea surveys, involving deployment of the EM sensors in shallow waters over a submerged cable with the power in the cable turned off/on, are described.