Many Flat Earthers insist that the earth must be flat because certain bodies of water are experimentally found to be flat. If the earth were a globe, they say, then the surfaces of these bodies of water would also be curved.
Even if these experiments get the right results (and it is a big "if"), they are inconclusive, because they overlook the flattening effects of surface tension in liquids.
From Wikipedia (my emphasis):
"Surface tension is caused by the attraction between the molecules of the liquid by various intermolecular forces. In the bulk of the liquid each molecule is pulled equally in all directions by neighboring liquid molecules, resulting in a net force of zero. At the surface of the liquid, the molecules are pulled inwards by other molecules deeper inside the liquid and are not attracted as intensely by the molecules in the neighbouring medium (be it vacuum, air or another liquid). Therefore all of the molecules at the surface are subject to an inward force of molecular attraction which can be balanced only by the resistance of the liquid to compression. This inward pull tends to diminish the surface area, and in this respect a liquid surface resembles a stretched elastic membrane. Thus the liquid squeezes itself together until it has the locally lowest surface area possible.
As a result of surface area minimization, a surface will assume the smoothest shape it can (rigorous proof that "smooth" shapes minimize surface area relies on use of the Euler-Lagrange Equation). Since any curvature in the surface shape results in greater area, a higher energy will also result. Consequently the surface will push back against any curvature in much the same way as a ball pushed uphill will push back to minimize its gravitational potential energy."
A still body of water a few miles long may be appearing flat only because of the effects of surface tension. Ths surface tension acts to reduce surface area as much as possible, and hence acts to eliminate surface curvature as much as possible. In a sufficiently small area, surface tension will keep the water looking flat.
Flat Earthers need to explain away this possibility, or else their experiments are inconclusive at best.