Exposing MREs to extremes of heat and cold reduces their shelf life - the lower the temprature the longer they last, freezing/thawing severely reduces their shelf life.
This is a good site for information on MREs -

http://www.mreinfo.com/mre-longevity.html

I found it helpful anyway ... at the end of their life they can still be edible as long as the packets don't swell up or the contents smell ok (though the vitamin content may be long gone).

As far as water storage goes, I'm still looking for the definitive answer re untreated tap water storage life. If anyone can give an answer I'd be interested. Still, if you were desperate for a drink, how bad could your stored water get ...