At the highest point, water does not fall out of the bucket rotated in a vertical circle, because weight provides the centripetal force.
This is an example of centripetal (circular) motion. Any net force causing uniform circular motion is called a centripetal force. The water stays in the bucket because of inertia. In order to make something move in a circle, you have to use force (a push or a pull). The force of the water pushing on the bottom of the bucket (centripetal force) will be stronger than gravity and the water will stay in the bucket. The weight of the water due to which water can fall is used up in providing the necessary centripetal force and the water does not fall.