One of the funniest things about the ocean is the fact that its surface is curved. We tend to think about water forming large flat sheets, but the surface of a large body of water is not actually flat at all -- it follows the curvature of the Earth.
Because of the curvature of our planet, the distance between you and the horizon when you look out over the ocean depends on your height above the surface of the water. The following diagram shows you how the distance to the horizon can change depending on the height of the observer:
So the distance to the horizon depends on the height of your eyes above the water. If your eyes are 8 inches (20 cm) above the water, the distance of the horizon is about 1 mile (1.6 km) away. A rough formula for calculating the distance to the horizon is:
SquareRoot(height above surface / 0.5736) = distance to horizon
where "height above surface" is in feet and "distance to horizon" is in miles. If you are 6 feet tall and standing right at the water's edge, then your eyes are about 5.5 feet above the surface. The distance to the horizon is:
SquareRoot(5.5 / 0.5736) = 3 miles
In metric, the equivalent is:
SquareRoot(height above surface / 6.752) = distance to horizon
where "height above surface" is in centimeters and "distance to horizon" is in kilometers.
These links will help you learn more: