According to the pilot in command, after night refueling at a riverside heliport, the helicopter was being repositioned to another spot to await passengers. The pilot in command was in the left seat, and the pilot flying was in the right seat. The pilot flying lifted the helicopter into a hover, facing west, toward the river. The pilot in command cleared the area left and right, and the pilot flying moved the helicopter slowly to the right. As the helicopter passed a helipad that jutted into the river, the pilot in command had difficulty judging "how far forward or back we were" since there were "no lateral position lines painted on the helipad deck to judge...position." As a sea wall came into view, he realized that the helicopter was drifting backwards, and stated "don't come back any further." Immediately afterwards, he felt a vibration, then took over the controls and landed the helicopter. A postflight inspection revealed that the tail rotor had impacted a fence. The pilot flying reported environmental conditions as "dark, drizzling rain, northerly winds 8-10 knots, [and an] unusually high water level of the...river."