Whether on the river, on the sea or in the mountains I always carry a group shelter. Not just for emergencies and dunkings but for the instant warm and sheltered lunch spot they provide. Remember though that without food the body can not warm itself. So get out of the wind and rain and be amazed at the speed at which it warms up in a group shelter, fill your bellies and you should be ready to go soon. If someone gets so cold they stop shivering then you need help and they should not carry on, cold blood in the extremities can be forced to the body core with disastrous results.