A seaplane is a plane that can take off from water and land on water — a lake, a river, or the sea — instead of a normal runway. It still flies in the air like any other plane; the special part is the take-off and landing. Some have floats under the body, like fat skis; some have a body shaped like a boat. People use them where there is a lot of water and few airports: islands, wild lakes, a coast with no long strip of land. You will not see one at a big city airport every day. The word is just sea + plane. People say Winston Churchill may have coined it, which is a nice story if true, but the meaning does not depend on that. Do not stretch it to mean any plane that merely flies over the sea — that is still an ordinary plane.