The origins of this flat earth madness are worth looking into. It's not based on any scientific critique, but in a literalist reading of the bible. Essentially a tiny group of religious fundamentalists refuse to accept reality because ot contradicts their religious text.
There's an unfortunate optical effect which can further confuse people. Atmospheric refraction, if I remember the term correctly. In suitable conditions light will bend along the surface of calm water at just about the same rate as the curvature of the planet. Sometimes you can indeed see for miles and miles, and things will not disappear behind the curvature of the Earth.
