The percentage composition of any given compound is nothing but the ratio of the amount of each element present in the compound to the total amount of individual elements present in the compound multiplied by 100.
Water has 2 molecules of hydrogen
One mole of water is of 18.0152 grams.
And one mole of a hydrogen atom is of 1.008 grams.
So, 2 hydrogen moles weighs 2.016 grams.
Hence, one mole of water has 2.016 grams of hydrogen mole.
Therefore, the percentage composition of hydrogen would be 2.016/18.0152 = 11.19%.