Dalton's first theory states, that all matter is made up of atoms (which cannot be divided). Continuing with this theory in mind, Dalton also states that all atoms of an element are identical in MASS and PROPERTIES. Lastly, compounds are combinations of TWO or MORE different types of atoms.
With Dalton's reasoning, water contains two atoms of hydrogen and one atom of oxygen; All water molecules are exactly like every other water molecule, and have the same 2:1 ratio.
Hope this helps!
-Moiselle