Here’s a handy hint from a geek site I read called LifeHacker.
MacGyver Tip: Dishwashing liquid ice pack
"The detergent stays cold much longer and it can be re-frozen over and over. It also molds to your back better than ice."
The author talks about freezing Dawn liquid in a ziplock. This is a handy thing to remember when you need ice packs, assuming you have a freezer onboard.