Bond angle of a molecule is related to the electronegativity of the central atom. The more electronegative the central atom is, the greater the bond angle.
As electronegativity of the central atom increases, the magnitude of bond pair- bond pair repulsion increases leading to increase in bond angle.
Hence, oxygen, being more electronegative than sulphur asserts a greater bond pair- bond pair repulsion leading to a greater bond angle in water.
