eek! the idea of electricity near my feet in the water upsets me. i've used "Toe Warmers" by Grabber, which are small chemical packs activated by air. there is enough air in your wader boot to keep them going. stick them to your socks on top of your foot and you're good to go. you can find them at shops that cater to outdoor winter sports. they have a shelf life of about 5 years.

double-you, double-you, double-you dot warmers dot com