Funny, I've never noticed all the skulls in this movie (mushroom cloud, waterfall, etc) anyway...
The waterfall thing is a tough. Seems line the only way to do it - which is probably a pretty big job - is to somehow drop a cliff (in place of the waterfall) behind one of the shots (1:29:30 or 1:30:06- ish) where the vehicle is falling down one of the falls. This would go right after they go off the cliff. This shot would also have to be flipped. Anyway, this way we could see them fly off the cliff and hit the water and climb back in. Then we could cut the Ox and Mutt dialog, drop their asses again and again if we wanted to in quick succession. Or cut the three times it drops thing all together and go straight to them walking to the rocks after the third fall.
Easy for me to say, huh?