The elements which form water are oxygen and hydrogen.
Three characteristics of water to show that it is a compound are:
1) Each water molecule is made up of one oxygen atom and two hydrogen atoms. The proportion of hydrogen and oxygen atoms in a water molecule is always 2: 1 regardless of the source of water.
2) The properties of water (compound) are completely different from those of its constituent elements hydrogen and water. For example, both hydrogen and oxygen are highly combustible gases whereas water is non-combustible and is used to extinguish fires.
3) Hydrogen and oxygen in water cannot be separated by any physical means but can only be separated by chemical processes like electrolysis.