I prefer a backpack but it can be a messenger bag too.Being %100 waterproof is the priority for me.

This is the best I could find. Submersible to 1 meter for 30 minutes. Not a backpack or a messenger bag, more a briefcase style.
http://www.pelican.com/cases_detail.php?Case=1495

http://www.amazon.com/Pelican-1495-Black-Computer-Case/dp/B000KIDBO4

I have seen this one too while searching through pelican website.It is more like a military stuff, very solid but not my type.I have a lot of electronic stuffs, apparatus' and thick document files, so that 's why i am looking for a backpack model."Pelican U100 Urban Elite Backpack" model is my dream product but unfortunately 17.3" does not fit 😕 I wonder if they make any changes on their products, like expand U100's case size 2 cm more 😄

well... like submersible to water waterproof? or splash/rain waterproof?

My Messenger bag from Mission Workshop
is basically waterproof as long as it is not submerged..
but if the opening had some some of closing mechanism it would be waterproof 😛

but this "The Vandal" is humongus 64L try and email them about how waterproof it is for your needs.
it will fit in the maincompartment
http://missionworkshop.com/products/bags/backpacks/roll_top/large_vandal.php

eventually the "Fitzroy" but I don't know if the Freak ASUS ROG series 17" "17" size would fit in the laptop compartment.
but again. ask them 🙂 their customer service is fast and great 😉
http://missionworkshop.com/products/bags/backpacks/rucksack/fitzroy.php