Here's how I look at it, with example photos I took myself, and supporting diagram to illustrate the principle.
The Isle of May lighthouse, in the Firth of Forth, Scotland, photographed from onshore, at Pencraig viewpoint, near East Linton.
EDIT to correct malformed link;
End EDIT
Closeup/crop from this;
My camera height was 100m
The lighthouse tops out around 73-75m (the light height is classified as 73m, so let's assume the roof above the light glass adds another 2m or so)
IF the land and the seas around this Isle were truly flat, surely my descending sightline through the top of the lighthouse (100m downward to 73m) MUST meet the flat plane of the sea at some point? Simple geometry dictates this.
My sightline is not meeting the sea. There's only sky behind the lighthouse.
Conclusion; the land and seas, around East Lothian at least, are decidedly not flat.
Perhaps, OP, you could generate some photos or diagrams yourself, to show what you mean?