Generally, an average height person can see up to 4.5 kilometers (2.8 miles) at sea level. This is the best possible visibility at sea you can have, if your eyes are six feet above sea level and the weather is great.
However, things are actually a lot more complicated than that.
Visibility: Nominal Range and Graphic Range
There are two ranges of visibility: the nominal range and the graphic range. The nominal range determines the maximum visibility of an object in the best possible conditions. For example, the light of a lighthouse could have a 21-kilometer (13-mile) nominal range of visibility at night.
The graphic range, on the other hand, is much smaller. It’s the distance from which you can see something. You may be able to see the light from 11 kilometers (7 miles) or even 3 kilometers (2 miles) away, depending on the circumstances.
What we usually call “visibility”, is the graphic range. This is limited by the curvature of the Earth and affected by the height at which your eyes are. So you will have greater visibility from the deck of a tanker than from the cockpit of a sailboat.
Your range will also be influenced by the size and height of the object you are looking at. For example, a tall lighthouse on top of a cliff will be visible from farther away than a small motorboat.
So visibility is the combination of the graphic range and the nominal range. Your graphic range may be 4.5 kilometers (2.8 miles) on a clear night, standing on the deck of your motorboat. However, you may be able to see the light of a 15-meter (50 feet) high lighthouse perched on a low rock from a distance of 23 kilometers (14 miles).