There are more knowledgeable people already in the thread giving good advice, so this is just my two cents: July is a busy month in Venice, but most tourists in Venice are there to stay in mostly the same areas and tour the exact same things as one another. Take advantage of that and try to rent an apartment in the far reaches. Castello is largely off the tourist path. Cannaregio well north of the train station is probably also pretty quiet. I'd probably pick Castello, but this is mostly subjective - first, it's the farthest from your fellow tourists, who will primarily arrive from the far west side of the lagoon where the train station and the cruise terminal are, second, I've seen it the least, and third, you can walk half of venice without even having to cross over the very touristy areas.