When water freezes, it expands. The part of the ice above the waterline is the amount the volume increased when it froze. The water's weight does not change from liquid to ice state. Therefore the ice displaces the same amount of water when it is frozen as when it is a liquid, so the level does not change.

When water freezes, it expands. The part of the ice above the waterline is the amount the volume increased when it froze. The water's weight does not change from liquid to ice state. Therefore the ice displaces the same weight of water when it is frozen as when it is a liquid, so the level does not change.