FlatEarthers mention the curvature of water a lot. According to them, the curve of the water can never be observed, and thus, the Earth is flat.
The problem with such an argument is that they usually conclude that from a very small amount of water. Water in a glass or a bucket will practically look flat. Not because there is no curvature due to Earth’s gravity, but because the water is in such small amount so that the curvature is minuscule and impossible to observe, let alone with the naked eye.
Water seeks the lowest potential. A particle of water farther from the center of the Earth has a higher potential than the same particle closer to the center of the Earth. And they are always trying to go to the location with lower potential.
As a result, the surface of the water will form a curvature centered on Earth’s center of gravity. Every water particle that forms the surface of the water has the same potential as they are roughly at the same distance from the center of the Earth.
Since the radius of the Earth is relatively huge (6371 km), the curvature will not be visible in the case of the water with a small surface area, like in a glass or a bucket. Even if there appears to be a curvature, it is caused by other forces such as the capillarity action.
In the case of a container with the diameter of 10 cm, the curvature of the water inside it is only 0.00000002 cm. It is impossible for us to perceive such a tiny curvature.
The existence of a 0.00000002 cm curvature in a 10 cm wide container is a consequence of the spherical Earth model. So if these flatEarthers want to disprove the spherical Earth using the water in a glass or a bucket, then they have to prove there is no 0.00000002 cm curvature.
Calculation
Inference vs. Observation
Some flatEarthers demand observational or experimental proof of the numbers we’ve posted here. The following is our response.
The numbers were obtained from calculation. Those are the consequences if the Earth is spherical with the radius of 6371 km. The numbers were not obtained from an experiment or visual observation.
The party who did visual observation is, in fact, these same flatEarthers themselves. They argued that because water in a glass is flat, then it will also be flat no matter how large is the body of water. And thus, the burden of proof is actually in their hands: “What is your proof that there’s no 0.00000002 cm curvature in the glass you are showing us?” A visual observation is certainly not sufficient to prove it.
The numbers we are showing were taken from inference, not observation. The numbers were derived from other parameters —like the radius of the Earth— which was known from other observations and measurements.
Reference

 Gravitational potential energy – Wikipedia
 Inference – Wikipedia