A ship disappearing from the bottom first is an effect of Earth’s curvature. It is not a direct observation of the curve itself. We can only directly see the curve from a high altitude, not from near the surface.
Flat-Earthers present this reality as if it is a dilemma: 1. Earth is too big for us to see the curvature, but 2. We can see ships go over the curvature. It is a false dilemma. Ships disappearing from the bottom first is an effect of Earth’s curvature. Such an observation is different from directly witnessing the curve itself.
Observing the curvature of the Earth means observing that the surface of Earth is curving. Seeing it is easier if the observer is at a higher altitude. On the other hand, ships going over the horizon is a result of observation from close to Earth’s surface. Earth’s curvature is the fact inferred from the observation.
Flat-Earthers like to dismiss the appearance of ships going over the curvature by pointing out that the horizon appears flat. In reality, these two observations are the different observations that require different conditions.
Directly observing the curvature is easier if the observer’s position is higher, and taken using a wide-angle camera lens. Seeing ships going over the horizon is easier if the observer is closer to the surface, and using a long telescope or a telephoto camera lens. It is practically impossible to observe Earth’s curvature and a ship going over the curve at the same time.