One of the better ways I have found to add moisture to the oven while baking (mostly bread, haven't tried it with pizza) is to put a heavy cast iron skillet in the bottom of the oven while doing the pre-heat. Then when putting in the bread a half dozen or so ice cubes in the skillet. The skillet has the mass to hold the heat and slowly release the moisture from the ice cubes rather than adding plain water which evaporates too fast. Ice cubes melt about just right. I may have gotten this from an Alton Brown segment, works great. Remember to re-season your cast skillet though.