All I can suggest is, once again -
Find a vantage point to look out on the sea and note its height above sea level
Observe something out on the water which is of lower height, above its waterline or coastal line, than your observation position.
Let's say you're at 100m elevation, looking at a ship of 52m.
You must be looking downward at the topmost point of the ship.
You must be looking downward at any and every point on the water's surface.
If the water is truly flat, there can be no instance where you look downward at the topmost point of the ship and fail to see water behind and beyond it.
A descending line from 100 to 0 must pass through 52
A descending line from 100 to 52 must, if continued beyond the 52 point, reach 0. It cannot miss it.
If there is ANY instance where you see clear sky behind and beyond the ship which is lower than you, the sea CANNOT be flat.
100m observation point, 52m ship with twin yellow cranes at approx. 17km. Nothing but clear sky behind and beyond the topmost point. The sea cannot be flat.
Nothing to do with Tom's "sinking ship", no need to show ships "going over the horizon".
Proof found in near-field objects, well within clear viewing distance. Can show the same with observations of islands, lighthouses, other fixtures. Loads of examples.
In a thread filled with great explanations and answers regarding various phenomena, this is, in my humble opinion, the simplest and most succinct one.
I do suggest one modification to make it even stronger and more obvious that this image (and the accompanying explanation) proves an earth that cannot be flat:
Can you make a second image based on the first, of
what we would expect to see, were the earth completely flat. That is, color water "above" the horizon, as if the earth were flat and you were looking out on an expanse down TO that water. The top of the two cranes on the ship thus still showing water above them.
I am terrible at math and geometry, but my instinct is that the sea, which stretches for hundreds (thousands?) of miles from this point, would intersect with the sky at exactly the height of the vision of the observer. Is that correct? I.e., rather than a horizon line, a convergence of sea and sky always relative to your eyes.
Yes?