This is an experiment in progress-For a real canyoneering trip, this didnt work quite as well as the short time in the pool. My initial impression above was after the pool trial. I do still feel this is a good method, and my preferred technique.
I guess that it worked pretty good. The fly did leak if in the water a few minutes.
I used it again a couple weeks ago for short crossings.
I think that it needs refinement, but I do feel it MAY be the best solution for a few reasons.
1. lightweight ( you carry no extra gear)
2. keeps your pack and gear ABOVE the water
3. The pack had less drag while swimming- it acted like a boat hull
4. My pack did get wet, but did not fill with water-which saved me a few extra pounds, I am sure
5. Still had a narrow profile, unlike the tube, which was good for bushwacking and scrambling.
From my experience, here are things to consider
1. I definitely need to seam seal my pack fly- May fix the leaking, may not, I dunno
2. The thermarest needs to be lashed across the top and bottom of the pack, so that it doesnt twist under the fly and stays put. If it moves too much, the pack will get top heavy and tip. ( With a secure thermarest-this doesnt even get close to happening)
3. Dont rely on anything to keep your essentials dry, still use a good dry bag- inside the pack.
4. Have a section of rope that you can tie one end to the pack, with the other end , tie a loop so you can sling it over one shoulder and under the opposite arm ( SO you can swim with free hands)-If you have long swims.
5. I am not sure that the thermarest would even be needed if the pack fly was seam sealed.
6.Definitely experiment with your technique (whatever method you choose) so that you know what you are working with, BEFORE you go. That was Matt GIlbert's suggestion and a good one
This method definitely could use some experimentation- I dunno, maybe I need to just put a grommet in the bottom of my pack to drain the water and be done with it
One more suggestion-If you are canyoneering-be careful, let someone know where you are at, plan to take longer than expected, and GO LIGHT as possible, and you have to expect that no one will find you or rescue you if things go bad. Never underestimate the weather-a flash flood will probably kill you-.
"I'd feel better if we had some crampons. Oh, what the hell, let's go for it..." — Common climbing last words.