I have done pack minties many times over the years but not how those guys did it.
When I was green, we used just one hootchie per pack. Pack in the middle, fold the edges up to the top to overlap, then one person would twist clockwise and the other counter clockwise, bring the twisted parts to the top and tie ends together. Then you helped your mate do his. You then swam across with your rifle on the top so you maintained security. At other side, just unwrapped it quickly, duck shoved bit and kept going.
Nothing got wet. Did it once as a training exercise in a pool and tied four minties together and floated it from one end to the other of a 25m pool with a female member on top. She stepped on dry, got off dry, all our gear was dry and it took about two minutes each pack, tops.
Have also just shoved my pack into my bivvy bag, with the sleeping bag still in the bottom, rolled the top down, made sure the roll was at the top and swum that across. Worked a treat. Wiped excess water off with shemagh, duck shoved it and kept going. That was pretty cool.
I have a couple of good pack covers, wonder if you could just make a kind of coracle with them. Might give it a try one day.
Cheers
Bloffy