John,
Yes, they can. However, they re not specifically designed for going underwater. To use our standard Digital Temperature Sensor, you would need to enclose it in a water tight housing (i.e. plastic tubing, rubberized sleeve, etc.). Our sensors are sealed and enclosed in an aluminum housing. There is a plastic cap on one end and it is sealed with silicon on the other. We do not guarantee the seal to be water tight and if water does seep inside, it will damage the silicon chip and short out the cable connections.
The risk is minimal since our Digital Temperature Sensors are extremely reliable and inexpensive at just $25 each (List Price). As long as you seal them safe in a water tight skin somehow then you'll be fine.
We have many customers who monitor water and also furnaces. Typically they do not have problems. Occasionally they will however call to purchase a replacement sensor. When we discuss the situation it typically involves them melting a sensor in an oven or letting it get exposed directly to water. There is little else that can fail since it is basically a computer chip on a cable.
If you send me your full contact information then I will be happy to send you out a complete information kit with product catalog.
