The heat packs are the gel ones where you flex a metal disc that is floating around inside when you use it the first time. It hardens and becomes opaque after the first use and you'll have to boil it until all of it is clear to re-use it. 2 types are generally available, with & without the metal disk. Those without can be reheated v. carefully in the microwave.
http://www.roshgo.com/Merchant2/merc...c01/CTGY/00602
Some sort of fanny pack/vest with strategically placed pockets is usually used to prevent burns as you can't adjust underwater.
As for the instant ones, the problem is how the chemicals react with each other under pressure.