The liquid in the cold packs you squish and shake to make work is just water. Some types can be disassembled to take out the water. The ones I worked on had the solid materials in a pouch. The water was also in a separate pouch. I took the water filled pouch out and inserted the solid material pouch into a zip-lock.
I had previously weighed the cold pack and noted the weight in grams. The weight of the cold packs outer bag and water pouch was pretty insignificant so I calculated the amount of water needed as one gram per ml. This I noted on the baggie with a marker.
With the solid components of the ice pack in the baggie I just add the appropriate amount of water from a canteen, it needn't be dead-on accurate, and shake. It works.
There is no real requirement to keep the water in the cold pack during storage other than to keep the unit convenient to use. If your desperate for water, like if you have to keep intestines or burned flesh wet, the water from a cold pack can be extracted and use.
Also, after you use a cold pack, you can safely slit the bag and save yourself the weight packing it out as trash. The chemical involved is ammonium nitrate, fertilizer. Don't dump this solution near a stream or body of water because it can trigger a fish kill. But, within reason, it can be dispersed onto live soil and cause little, if any, damage.