Although in theory it sounds like an easy experiment, as others wrote, in practice it would be rather difficult. At three miles, the height of the Earth curvature in the middle is some 45 centimeters (around a foot and a half). Now you would need a perfectly flat water surface with no waves and no currents. But not only that. You would have to exclude that no garbage, fishing lines or nets, dirt, sediments, water plants, fish/plankton, or gas bubbles impact the buoyancy of your line over all the length. You would have to exclude any boat traffic in the proximity. You would have to account for the influence of the tide ... Read even more
