Hah, I tried earlier today to do these tricks into a none-raft, and ended up taking an 8 foot bump onto concrete.
Fair warning to anyone and everyone who wants to try this themselves: Even with a raft (which, oddly enough, is a fantastic safety device), you should NOT
try to do these things yourself unless
you are ready, willing, and above all else able to take a fall onto whatever surface you're doing it over.
While I hadn't mentioned it earlier, I've had about a decade of experience jumping off of things I shouldn't... So me going 8 feet on my back into concrete isn't the potentially life-changing screw-up it would be to someone who HASN'T survived 30 foot falls into the ground.
Basically, since I've gotten some messages through youtube from angry people who have tried something similar at home...
Unless you have what amounts to stunt training, don't do as the stupids do. I don't like living in a world where I have to say that to avoid lawsuits... But we do. Hah.