Water is a unique compound with particles that stick together in long chains, making it easy for light animals to balance on the surface.
The two different elements that make up water are hydrogen and oxygen, and the way the molecules interact affects how the compound behaves as a whole.
Oxygen particles are large and attract electrons. In water, oxygen shares a pair of electrons with two hydrogen atoms. Because of the attractiveness of oxygen, these shared electrons sit closer to the oxygen atom than the hydrogen atom.
As electrons are negatively charged, this gives the oxygen atom a slight negative charge and because the negatively charged electrons are far away from the hydrogen atoms, the hydrogen develops a slight positive charge.
This means that every single molecule in a body of water is slightly charged, either positively or negatively. The positive atoms are attracted to the negative atoms and form a chain in which each molecule sticks to the others around it.
This chaining is what allows small animals like ants and water boatmen to walk on the surface without sinking. It is also what allows water to move up a stem or a straw and explains why water forms round drops.