I wouldn't try it. Typical crockpots have low and high settings which range from 200-300 degrees. Those with warm settings would keep food warm at 150-170 degrees. Obviously too hot for drinking water and a possible fire hazard. Not to mention it would cost a fortune if continually on. Just search this site for water heaters and you'll find some interesting ideas that are much safer.
I have hung a space heater from the ceiling in my insulated hen house. The heater has a setting that it kicks on only at temperatures below 32 degrees. Once the air temperature rises, it kicks off again. Because my coop is insulated it stays pretty cozy -- between 35 and 40 degrees -- hence the waterer doesn't freeze. I feel this solution is really safe and comfortable for them.
Can something go wrong -- something can always go wrong -- but I feel I've taken the best precautions I can. This space heater was on sale at a hardware store for $20.00.