Unless the device was specifically made for this purpose, do not put battery powered devices in an airtight container, including a plastic bag. Batteries outgas over time and it becomes a potential fire hazard if not properly vented.
If this is a device made for that purpose, of course, that’s a whole different situation.
Usually the safest temperature sensor for any liquid is one that attaches to a non-powered probe. The probe goes in the water, the part with the radio stays outside. if you’re interested in that approach, see the following thread:
Temperature Sensor With Probe?
