Here's a repeatable experiment using a mirror and line of sight that anybody can do to prove that water, and thus the earth is flat.
The error in your logic is
assuming the earth is flat because water appears flat. I can post another image of a basketball appearing flat, but you wouldn't assume it was flat, would you?
Show me observable, measurable, repeatable proof of curvature gimp.
Check out the live NASA space station feed. Observe and repeat as needed.