What is the molecular formula of water?

Answer 1

H2O

Answer 2

If you want the simplest formula, then the answer by Ann is correct. However, a somewhat more complicated answer is that water forms clusters, due mostly to hydrogen bonding. Here is a site where you can read more about it.
http://www.lsbu.ac.uk/water/clusters.html
At 100 degrees celsius (steam), the structure is simple H2O molecules.