It’s surprising how close the actual horizon is. Of course, any object with a bit of height to it can be seen ‘hull-down’ at greater distances, but for the sea / sky interface itself, the formula is to multiply the square root of the height of your eye by 8/7. If you can’t be bothered with this, just remember that seated in the cockpit of a small yacht at 4 feet, the horizon is a bit less than 2.5 miles off. Standing on the side deck at 9 feet, it’s 3.5 miles, and perched up on the boom at 12 feet, you get an extra half-mile to four miles. Not far is it?