How far can man see to the sea when he is standing on the beach?

Ordinarily, the farthest visible big wave might be half a kilometer to a kilometer away. On a calm day, someone with eyes two meters above the water could see water out to the horizon, where his vision would be tangent to the Earths surface and at right angle to a radius. This makes a right angle with two sides of about 6371 kilometers

and .002 kilometers greater. Using Pythagorus, horizon^2 + 6371^2 = 6371.002^2 so horizon = 5.048 km.

There is a far simpler equation to arrive at a solution in miles, this it the square root of 1.5 times the height of a human above sea level (in feet), using the sum above, 2 meters (6'6") * 1.5= 9'9", the square root of which is 3.146 = 3.146 miles, 5.048Km = 3.136 miles, taking in to account that's the height of his eyes not his head, i would say this equation would suffice and is far simpler and easier to remember.