That argument is bogus. View directly in front of you is obscured by refracted water and air. Try looking that same distance - 3 to 6 mi - either left or right and you will see ships very clearly. If the earth were a ball, then all ships in all directions would be going over a curve, not just the ones out in front of you.
Take a Nikon P900 to the ocean and watch a ship "disappear" then zoom in. It is still there.