Arguments about the flatness of the earth should be implictly addressed to those who are agnostic on the issue. Otherwise we just end up begging the question with each other.
In my view, whichever side presents the arguments most likely to sway a reasonable agnostic, wins.
I think that the following argument will sway any reasonable agnostic into believing in a non-flat earth.
Here is a simple and easy method for demonstrating to yourself that the earth is not flat.
All you need is a long-distance phone service and a contact (friend, relative, etc.) in a far away land.
(1) Remind yourself that the sun sinks in the west, and rises in the east every morning.
(2) Infer from (1) that when it is night, the sun is in the region beneath you.
(3) Call your contact when it is night where you live, and daytime where they live. Ask them if they can see the sun in the sky above them. Unless it is cloudy, they will say "Yes".
(4) If they say "yes", then since you know the sun is beneath you, your contact must also be beneath you. Moreover since you know they see the sun in the sky above them, you know that their sky is in a different direction from your sky.
(4) is impossible unless the earth is not flat.
This simple experiment is superior to all the "water-based" experiments promoted by Rowbotham and his followers, because those only prove the flatness of small stretches of water, whereas this proves the roundness of the Earth over a very large distance.