I have used these in the tropics. Not much good for retaining water for long periods as they lose water through evaporation (hence keeping the remaining liquid cool) You have to keep topping them up after 24hrs or so from another source. We used them for treating heat exhaustion, although they are fantastic for keeping tins of beer cold,,,,,far more important.
That was in the early to mid 90's. don't know if these are still issued though.
Definately worth having if you need to keep something cool.