This document provides 10 arguments for why the Earth is flat rather than a sphere:
1) The horizon always appears flat regardless of altitude and objects should disappear over a curved horizon.
2) The horizon rises to meet the observer's eye level at all altitudes, which would not occur on a globe.
3) Natural physics dictates that water finds and maintains a level surface, which would not occur on a spinning globe.
4) Spherical trigonometry calculations of surface curvature have not been observed with experiments on standing bodies of water.