Seeing the Curve

When swimming you can notice the effect over the width of a small lake:

But if you want to see a noticeable effect over just two meters then it depends on what you mean by “noticeable” (eg it would be a trivial high school exercise to calculate the largest radius at which a two meter tangent line ends at least one mm from the corresponding arc).

Source: (1001) Alan Cooper’s answer to How close to the Earth’s core would you need to be to see an ‘uneven’ water level?Not regarding temperature etc making it impossible, how deep would you have to dig to see a noticeable curvature in, lets say a 2m length of water? – Quora