I've heard of people storing important documents in a plastic folder and placing it in their freezers, just make sure to take them out if defrosting or if there is a power failure. The best option is a fire safe box from Office Depot etc.
The best passport cover i have found is in CVS at the photo desk, usually they are on the counter, a clear plastic cover costing 99 cents each. The passport shuts over fully with this type cover and no need to take it out of the cover at passport control/Immigration- at least not for me since i have been using mine.
The are other covers available at a few dollars more, i tried them and they are cumbersome and don't close the passort fully and the passport does need to be taken out of the cover at passport control/Immigration.
I bought 10 at CVS and gave out some to my freinds.