my top two choices for water proof messenger backpacks.
http://www.chromebagsstore.com/warsaw.html
http://seagullbags.com/backpack/
and here are the rest of the chrome bags.
http://www.chromebagsstore.com/laptop-bags.html
http://www.chromebagsstore.com/messenger-packs.html
i've seen this one in action.http://www.chromebagsstore.com/soyuz.html it works great, but a little on the small side for me.
all water proof. some of the chrome bags have laptop sleeves. the seagull you'll have to get a flap pocket added and probbaly a padded sleeve for security.