Place them into an oversized ziploc bag. Seal it but don't remove all of the air. Place the closed ziploc bag and docs into an even bigger ziploc bag. Seal it as well. Leave also a pocket full of air, so that the big within a bag can move around.
Take this nest of bags and wrap duct tape around the entire thing. Not once but twice so the bags and the documents are effectively waterproof. Place the taped bags and place them in two heavy plastic trash bags. Seal them up well and tight.
Finally tie the bag of documents to an emergency life ring (illustrated below).
There ya go. Your documents will float in a flood. But even if they don't they'll stay nice and dry.