Hi RobertNielsen,

I prefer the Aloksak Resealable Polybags,

The small bags in the photo will carry about 400ml of water reasonably securely and have other uses as well if they are used to keep other things dry such as in the photo below. 2 small or 1 medium Aloksak bags will easily fit inside an BCB minimess tin PSK with the rest of the survival kit. They fold up very small and they are pretty tough. When carrying water it is important to ensure that the seal at the top of the bag is sealed perfectly though before carrying water inside.