In our old camp, we used a Freeze Alarm.
http://www.controlproductsonline.com...p-17-l-en.html
This device operates on AC, but has a 9 volt battery backup (and you can use 6 D cells to get a longer battery life). It does require a landline to operate. This will not sense the temperature of a water line but could be used to monitor the temperature in the crawl space. It also notifies you if the power is out or if the battery is low and you can call into the device to get this information.