You see some of us know the Earth has seas, land and mountains. In other words the surface is not even. But we know the seas are even and if we use the seas as the standard surface measurement the science can work out more accurately the distance between areas and work out its bend. This of course it too complicated to your man to understand.
Flatearthism has no doubt many good arguments to explain how it could work. But it also depends on there being a long conspiracy to hide this from the world, with many thousands of people all involved in making sure this 'secret' never came out.
Best of all however, is that the biggest problem for flatearthers is to keep the edge of their Earth out of sight from anybody when all they need to do to convince all that the Earth is indeed flat by flying a plane, a spaceship, a balloon or a drone over the edge of their flat Earth taking videos or photos of it. Now that should convince everybody. So, who don't they do that? I tell you I for one would have to accept that as the only real proof for a flat Earth.
Regarding these points:
If water measurement is preferred and a 23 foot ship disappears after 6 miles out
(actually Nikon P900 cameras have shown this not to be true but let's assume it is.)
why isn't there a visible 23 foot drop on the ocean or lake horizon on the sides?
After 10 miles there should be a 66 foot drop.
When viewing 10 miles of water horizon, ocean or lake, there should be a 66 foot drop on the sides. The horizon should not be a straight line but it always is.
Also, shouldn't the drop should be visible when viewing skylines that are waterside, such as Boston, New York, Chicago, or here is Miami:
The buildings do not start to lean away from each other as they go "over the curve". Can you see a 66 foot drop that causes the buildings to lean?
The buildings remain level just like the water.
Maybe these aren't the best photos. Are there any photos of any skyline you can provide that demonstrate the curve with the 66 foot drop after 10 miles?
As far as the longstanding conspiracy and how many people are involved, that's an understandable question. Yet we know of so many other cօռspιʀαcιҽs that have been almost universally maintained for a long time:
The earth spins 1,000 miles per hour
The earth rotates around the sun
Man came from apes
Man landed on the moon
Masks protect from covid
Social distancing will protect you from viruses
The shots that don't prevent infection or transmission will protect others
And regarding the edge, consider that no one is allowed to pass the 60th parallel unescorted by a unified governmental consensus. Why not?
This video explains further about that: