http://www.tractorsupply.com/en/store/api-all45%3Bseasons-electric-heated-pet-bowl-5-qt
That is what it is intended for.
Although wattles can get wet,they are not porous so they wont hold water and freeze.
Edit: i should have said absorb the water
My observation based upon the options that were give-nth to me at the time
water bowl $20 50watts. $5.76/month x6 =$34.56+20=$54.56 has a thermostat so electric usage maybe lower
tin can $15 75watts $8.64/month x6 = $51.84 +15 =$66.84
Based on $.16 a KWPH rate