well i have read all that replied to the question. but no one even thought about the pocket hand warmers, not the battery powered one. they are chemical filled and last about 12 hours. I am not saying to put them in the water, but under the container. i have used them when i worked out side in the cold and placed them inside my clothes. and with it under my coat i stayed warm for hours. why not try one under a ceramic tile which may work even better... just a thought
