When dealing with Stoichiometry, it is best to convert to moles. Then you have to determine your limiting reactant (reagent). Do this by creating an if/then statement about your reactants. If I have X moles of H2, then I need Y moles of O2. (In this case, you need half as many moles of O2.)
If you have that much or more O2, then H2 is limiting. If you do not have enough O2, then the O2 is limiting.
Use the limiting reactant to determine how much product (H2O) you should make. Convert your moles of product to grams. Then find the percentage of what you made to what you should have made.