: I blew up an empty wine cask "sack"....

I use these all the time for whitewater, rather than custom-fitted PVC
float bags.

1) You have a a degree of redundancy - it takes four or five to fill each
side of the boat behind the cockpit, so if one bag bursts you only lose
10% of your buoyancy rather than 50% if one of the PVC bags bursts (I use
the 3 litre wine bags)
2) They do rupture catastrophically from time to time, but if you use a sligthly larger number of them, not fully inflated (a) there is less risk of failure and (b) you can just blow the remaining bags up a bit harder and be
no worse off.
3) It's easy to add or remove bags as you vary the amount of kit you need to carry in the boat - put small things in the boat and blow a bag up to hold it in. All in all a lot more versatile than the normal one-big-bag-in-each-half
arrangement.
4) As we generate lots more of these than I need (wine boxes are perfect for wine for cooking, even if you turn your nose up at drinking the stuff) and they store quite well flat, I always have several kicking around in the car in case I need a replacement - or want to lend a few to someone else who bursts a bag.
5) We've had several bought (and not cheap) PVC bags fail - usually in the cold as the pipe stiffens up and tears out of the bag.
6) They smell great as you let them down to fit more gear in ...

All in all, even if PVC bags were free, I think I'd still use the wine bags.

I used to use PET drinks bottles for buoyancy - these really are robust and don't burst, but they don't pack as tight, and do weigh a fair bit more. But in a sea boat you can cycle them between use for carrying water and use for carrying air. They are also useful in a whitewater boat in that any nasty fish hooks that you find (they are often caught in branches at eye-level waiting to cause a nasty injury) can be popped safely inside a PET bottle where they won't puncture anything...

Andy