I used heat sealable packcloth to make scuba bag prototypes. I used the heat sealable type because I didn’t want to sew a bunch of different patterns while trying them out. A regular household iron seals it fine but I used a sealing iron for RC plane wings to add precision.
The seam is strong and water tight. I’d think heavy duty stitching would be stronger, but for many purposes the heat seal is strong enough. Since it’s just regular packcloth with a shiny glue side added, it has the same durability expected from packcloth. As a test I filled a bag with about a gallon of water and hung it from the garage ceiling. After several months it still didn’t leak. I’ve also pulled on seams hard enough to turn my knuckles white without it tearing, though my suspicion is that if I tried hard enough I could rip it with the right leverage.
The material is pricey compared to regular packcloth, but it saves time on sewing and lets those that don’t know how to sew work with a tough fabric.
[Here's a high-res photo of Aaron's bag - Mark Frauenfelder]