Looking out from onshore, at one or more ships or boats, the observer can only be, at any one time, in one of three states;
A. Below the highest point of the boat or ship
B. Level with that point
C. Above that point.
For an observation from below the highest point of the ship, then regardless of whether the water is flat or curved, the sightline will always meet the sky, since it will always be an upward sightline, away from the water.
For an observation where the observer is level with the top of the ship, the sightline will, again, meet the sky, not the water, whether flat or curved.
https://imgur.com/QeRAtIqThe situation where we actually see the difference is where the observer is above the highest point of the boat or ship. There can only be three possible "views" of a ship on the sea, when an observer is above the upper structure of the ship. Let's call them Type 1, 2 and 3
Type 1 - you can only see water behind the ship. The upper structure of the ship is below the horizon.
Type 2 - the waterline of the ship is below the horizon, but the upper structure is above it.
Type 3 - the waterline is at the horizon, and the upper structure is above it.
I don't think there's any other options, unless you invent a flying ship.
You can either - see the various types where they are at various distances from your observation point, or - you can (try to) persuade a chosen ship to remain at anchor, while you climb or descend.
You can see all three in scenario A (fixed observer height, various ship distances) in YouTuber Chang.'s video here -
Title "Flat Earth Observations", uploaded 11 Sep 2019. Channel "Chang."
Type 1 is at 3m34s, Type 2 is at 3m10s, and a selection of Type 3s in the first 3 minutes or so. All 3 types are shown after the 4 minute mark.
Here's Type 1, 2 and 3 drawn freehand, along the top of this graphic.
https://imgur.com/6BBfwCzI can illustrate a side view of Type 1 on a flat sea, with sightline (green line). Can anyone illustrate a side view of Type 2 or 3, assuming a flat sea? Can anyone fill in the question marks?
I can illustrate all three, and how the sightlines work with them, on a globe, and these are shown along the bottom row of the graphic.
Can anyone fill in the question marks with the appropriate diagrams for a theoretical Flat Sea Type 2 and Type 3?