@janbb It only depends upon how far above the water you are. I live near lake Ontario, elevation about 250 ft. If I am on the shore of Hamlin Beach and am about 10 ft above the water the horizon is ~ 4 miles away. If I am on the shore in Miami about 10 ft above the water the horizon is still about 4 miles away. What is really neat is you can see and understand why the bottom of ships are cut off when they are in the distance. If you are 10 ft above the water the horizon is 4 miles away and if the lower 20 of the ship is missing that means it is about 1.23 x Sqrt(20) = 5 miles past that or 9 miles away. With that info you can dial in your artillery rounds. :-)
On a clear day, for an experiment try to watch a ship slowly sink out of sight before it fades away.
Here’s a practical, real world example. If I have a tower on a 2000 ft high mountain, say in the Southern Tier, and want to talk with a city with an elevation of 500 ft, Rochester NY. I figure a difference of 1500ft. The horizon from the mountain top is 1.23 x sqrt (1500) = ~50 miles away Since Rochester is 60 miles away I need to add a tower at that end to guarantee line of sight. How high should it be? I want 10 miles so solving for 10 miles I get 10/1.23 => 66 ft tall tower. I’ll use Rohn 25G with 6 – 10 ft sections and add a 6 ft aluminum mast. . Perfect communication 24/7 direct line of sight. Or I can mount the small receiving dish on the 7th floor of a building with an unobstructed view of the antenna. Stealthy and easy.
And it was from 7th grade science class!