Paul, I know you might have been talking about purely kayaking (?) but I've got to say that in the UK , where our mountains (unfortunately) lend themselves to hypothermia we don't go a lot on heat packs of any kind. UK treatment insitu is usually get casualty in shelter and heat it, or better still,get casualty in sleeping bag and join them - Shared Mutual Body Warmth. The latter is particularly welcome if one of the parties is a nubile young lady from a Signal regiment <img src="images/graemlins/tongue.gif" alt="" /> (sorry ladies!).