There are 2 reasons why boats and planes might disappear from sight.

Reason 1: The biology of the eye. How good are our eyes at seeing? Eyes are good at seeing things only to a certain distance. If I am at one side of a football field, I can see a ball very well if it's on my side of the field, but the further and further away it gets, it is harder to see. But if I was a hawk, I would be able to see this ball even if it were placed 2 football fields away. A hawk can probably even see a small mouse at that distance. Why can some animals see things better than others? Our eyes have evolved and developed over thousands of years but some animals adapted better eyesight than ours because they need it to survive.

Reason 2: If we are looking at a flat landscape, things like boats and planes may disappear over the horizon. This is not due to our poor eyesight, it's because the earth is a sphere (like a ball). In general, the curve of the earth is so small that we don't usually notice that we are living on a curved surface. In fact, it was a major discovery in science when the earth was found out to be round, not flat, because it's not that obvious! It's only when we are looking over very long distances that we notice, such as watching a boat on the horizon. The crew on the boat will not notice that the earth is curving, but the people watching will see the boat slowly dip down below the horizon as the boat follows the curve of the sphere.

I am not sure what you mean by 'disappear'. Usually when a plane 'disappears', it crashes, and nobody has found the crash site or knows where it went down. Ditto for a boat that sinks. Airplanes and boats do not simply vanish, though - we just don't know their fate, but whatever it was, it wasn't good (if it were, then the plane or boat would have reached its destination instead).