I don't understand your example. We do see a cup or a bowl can be filled and overflow.
Do the seas overflow? No? Great, got your answer there.
Also, could anyone visually see the ocean like a cup, even at 100 years ago?
100 years ago, easily. Given that there is a limited space for oceans to occupy, and 100 years ago we were easily aware of that.
More than 2000 years ago, why not? There are lakes of which we cannot see the other side, but we can know they are lakes because as nomads we can travel around them. There is no specific reason why a nomad wouldn't perceive a sea as a specifically big lake. All it would take is a person to see the earth as a disc with a finite circumference.
edited to add: And we have clues from the bible that the Jews saw the earth as having a finite circumference.