The underwater electric field has been measured during a 10-month period by two three-axis electrode systems in shallow water off the west coast of Sweden. The long-term performance of the systems has been evaluated. One out of the six channels failed at the end of the experimental period. Data were collected in 10-minute intervals, at most once per hour. The measurement covers frequencies from a few mHz up to 20 Hz. The natural background in this frequency interval has been characterized. It contains sources of various origins, where man-made sources dominate at the higher frequencies. At lower frequencies micropulsations are important. The two systems were separated by approximately 720 m. The correlation between the signals in the two systems is high, which implies that non-local sources constitute the main part of the received signal. One system may thus be used for reference filtering, where a large part of the background can be removed.