Imagine standing in a tall corridor or hallway, which is so long, that you can't see the end. You're standing on the floor observing the end of the corridor from a low angle, and that means, that you can see less of the floor than you can of the ceiling. If you could float up to the ceiling and observe the end of he corridor from there, you would see more of the ceiling than of the floor. But back down to observing the corridor from the floor: If you see a person walk away from you down such a corridor, you'll will notice, as this person moves farther away from you, that first the feet wil be visually cut of and then the lower legs and so on until you can't see the person anymore. That is perspective in effect - the way our eyes see the world!
Imagine removing the walls of the corridor completely and replacing the floor with the sea and the ceiling with the sky. You'll see the exact same effect happening when a boat disappears out to sea bottom first: You see less of the sea than of the sky because of the low angle of perspective, you're watching the scenery from. So it's because of the low angle of perspective we're watching the scenery from, that objects, which are lower than the horizon are visually cut of at the so called vanishing point (where the sea / ground visually meets the sky forming the horizon).
There's a limit to how far our eyes can see. If the angle of perspective gets too shallow / low then things will start to visually blur out (a gradual compressing of the scenery observed) and eventually disappear the farther you look into the distance.
I hope, this can help in understanding, why we can see boats disappear bottom first. As "Phuket Word" (recommended YT-channel) usually says: "Pespective IS the imaginary curve!"
The theoretical Flat Earth map and the UN logo.
If you think, I'm religious, you're wrong.
If you think, I'm an atheist, you're also wrong.
I seek the absolute truth - no more and no less!
"The one, who seeks the truth,
is also the one, who reveals it."
*Jesus said: ”Beware of false prophets,
which come to you in sheep's clothing...”