Everybody beleive the canvas is not waterproof ; he could be, but the photographer will certainly go undercover before.
Last month, I covered the 65th anniversary of the liberation of Auschwitz extermination camp in Poland. Snow and 23°C temperture below zero.
My only concern was to let my F1X down on the ground. Not for humidity but for dirt patches he got underside.
Back home I bought a rain cover in a military surplus store, to be put in the back zipped pocket when I am on assignment by hazardous weather. Very useful and easy to operate.
I consider my F1x as the best 'basis' ever, like a combat aircraft equiped depending of the mission.