The curvature is visible. That's why you see the tops of ships at the horizon before you see the rest of the ship.

That was answer #2. However, technically/literally this is NOT seeing curvature and it is important to recognize that. The curvature is NOT visible while watching ships, as the horizon always appears flat.

The observation you mentioned is interpreted and the existence of curvature is inferred from it. This is very different than seeing (or measuring) the curvature inferred to be there.

That's like the most basic observation one can make and what I learned as a little child when it came to the topic of the round earth.

You, me, and most everybody else too. We are taught such things as little children, long before we have the capacity or freedom to critically evaluate or refute such interpretations.

It truly baffles me how anyone can deny that.

I haven't come across many who do deny it. The observation of ships over the horizon is demonstrable and repeatable. In flat earth research, it is most often the interpretation of the observation at issue, not the observation itself. Denying what is trivially demonstrable and observable is silly / borderline crazy.

Why would you ever claim that nobody has done this?

Because that is the conclusion that i have arrived at after a lot of study on the subject. There is not now, nor ever was in history, a procedure (or further, measurement obtained from such a procedure) to directly measure the convex curvature (described by the globe model) of the surface of still water under natural conditions. In fact, all the procedures for doing so in hydrostatics over three plus centuries show clearly that the surface of still water (barring negligible surface tension artifacts) is always flat, level, and horizontal and cannot, by its very nature, take such a shape at rest without large amounts of energy to fight against water's inherent properties.

https://www.astro.princeton.edu/~dns/teachersguide/MeasECAct.html

This is again not a measurement of the surface of still water (direct or otherwise). Such calculations (going back to the first recorded, eratosthenes) absolutely depend on the sphericity of the world in order to be meaningful. If the earth is not spherical, as these calculations assume/require, then the numbers they produce are meaningless and the cause of the observations (used to infer such sphericity) popularly believed and taught is wrong.

The curvature is visible. That's why you see the tops of ships at the horizon before you see the rest of the ship.

That was answer #2. However, technically/literally this is NOT seeing curvature and it is important to recognize that. The curvature is NOT visible while watching ships, as the horizon always appears flat.

The observation you mentioned is interpreted and the existence of curvature is inferred from it. This is very different than seeing (or measuring) the curvature inferred to be there.

That's like the most basic observation one can make and what I learned as a little child when it came to the topic of the round earth.

You, me, and most everybody else too. We are taught such things as little children, long before we have the capacity or freedom to critically evaluate or refute such interpretations.

It truly baffles me how anyone can deny that.

I haven't come across many who do deny it. The observation of ships over the horizon is demonstrable and repeatable. In flat earth research, it is most often the interpretation of the observation at issue, not the observation itself. Denying what is trivially demonstrable and observable is silly / borderline crazy.

Why would you ever claim that nobody has done this?

Because that is the conclusion that i have arrived at after a lot of study on the subject. There is not now, nor ever was in history, a procedure (or further, measurement obtained from such a procedure) to directly measure the convex curvature (described by the globe model) of the surface of still water under natural conditions. In fact, all the procedures for doing so in hydrostatics over three plus centuries show clearly that the surface of still water (barring negligible surface tension artifacts) is always flat, level, and horizontal and cannot, by its very nature, take such a shape at rest without large amounts of energy to fight against water's inherent properties.

https://www.astro.princeton.edu/~dns/teachersguide/MeasECAct.html

This is again not a measurement of the surface of still water (direct or otherwise). Such calculations (going back to the first recorded, eratosthenes) absolutely depend on the sphericity of the world in order to be meaningful. If the earth is not spherical, as these calculations assume/require, then the numbers they produce are meaningless and the cause of the observations used to infer such sphericity are in fact attributable to another cause.