The single most important factor will be the waterproofing of this light. For this reason I would very strongly recommend you get a dive-rated light, then you will KNOW it won't fail. The other requirements are all secondary to its total reliability, and the quickest way to compromise that reliability is to flood it. You will always have a sneaking doubt about a light that is merely "dunk-proof".
I would respectfully question the run-time as a matter of serious importance, unless you are going to be on the river during actual darkness, which I rather doubt (?). Presumably this light will be mostly for camp use at night, when you would have an opportunity to change the cells if needed. As you are rafting, presumably a bit of extra weight is not a problem as you won't actually be carrying it on your back, so taking extra cells will not be a problem. You will need a water-tight case for them.