Because of its density. Density is mass divided by volume. This was proved during Galileo's time (mid 1500's) when someone proved that shape doesn't affect whether or not an object floats but density does. Water is more dense, that's why. Real ice floats but Hollywood movie ice doesn't float since it is a prop and it just sticks to the bottom. So hope that answers your question!
Scientists today still wonder why water Is the only material to expand and become less dense when solidified. Usually solids become smaller and denser when solidified/frozen. But no it becomes less dense causing it to float