Most substances become denser when they freeze, but water is unusual. As it forms ice, hydrogen bonds arrange water molecules into a crystalline structure that takes up more space. That makes ice less dense than liquid water, so it floats.

This quirk is more than a curiosity. If ice sank, lakes and oceans would freeze from the bottom up, devastating aquatic ecosystems and changing the climate. A small molecular oddity helps make life on Earth much more stable.

Reference:

Wikipedia: Ice

https://en.wikipedia.org/wiki/Ice