If you ever plan to go in such as bad condition that water can be with all your fency material, you shouldn’t leave without something good to carry it in. Rain is an issue, most bag offer a protection with it, like Lowepro, you can get water-resistant or water-proof from this compagny. Still, the waterproofness of their products is questionable: if I ever happen to go in a drain, not that it’s legal but everything happen in life, will it keep everything safe? : I don’t think so!
What’s the option?
SealLine

I got my hand on a Boundary bag 35L for about 60$ at MEC.
The first thing you’ll realise is how this could be big when it’s open, but don’t be fooled, it will get smaller when you close it! The closing system is a bit strang but that’s what keep it so well close to the exterior. Roll the side and join the two clip and you’re done. It is more than easy to say the bag is proof to everything since the air can’t comes out from it.

Sadly, do not expect to throw your things in there like a regular bag, you should throw your bag into this one if there’s too much water or vice versa. I prefer putting my bag in it, so I don’t cut it.. Also, the color is by far not really ‘’stealth-proof” but I only found yellow, orange and green. Through it can be folded in your pack before you enter any place. It’s not easy to open in restrained area as a drain can be.. and you don’t want to drop it neither, but it is doable. I thing I just took it a bit too big, my fault, my bad but I like the bag.
The straps looks a bit cheap, but in fact they do the job well. The bag is confortable when you keep the air inside it, otherwise it’s a tracking pain for any back. Don’t count on the hips straps to do the job, they don’t work as well as the lowepro do (at all). Fortunatly, the tissus is resistant enough for what I use to carry: books, cameras, clothes, laptop…

Overall, this bag is not made for photographer but it works better than most bag for photographer (talking about waterproofness here). It can be carried, folded easily with anything. That’s a nice add if you wish to protect your stuff away from the bad thing that keep your body alive.