Near water does not necessarily mean near an Ocean or Sea, or lake large enough to have similar properties with regard to the horizon.
There are in fact many benefits to living next to a river or lake instead of the coast. A large one is that the water is mostly potable, instead of largely not. I would hazard a guess that most ancient coastal communities were actually ancient coastal river communities, and that rivers are actually the thing humans lived next to (there's some indication in the abstract here[1] that this is the case). If that's the case, proximity to water is a poor indication of how many people would have seen ships sailing over the horizon. As to how obvious bases of trees would be over large lakes, or how likely large lakes would be compared to the norm, I don't know.
This is largely about water as a home to food sources. Rising sea levels are believed to have covered the majority of Mesolithic cites. For populations constantly on the move it’s likely a majority of adults saw the ocean or a sufficiently large lake to show this effect.
Realizing it was evidence of a round earth is of course a different story. My guess is most people simply never considered it.
There are in fact many benefits to living next to a river or lake instead of the coast. A large one is that the water is mostly potable, instead of largely not. I would hazard a guess that most ancient coastal communities were actually ancient coastal river communities, and that rivers are actually the thing humans lived next to (there's some indication in the abstract here[1] that this is the case). If that's the case, proximity to water is a poor indication of how many people would have seen ships sailing over the horizon. As to how obvious bases of trees would be over large lakes, or how likely large lakes would be compared to the norm, I don't know.
1: https://www.nature.com/articles/s41467-019-08366-z