Good Water Sensor for Vista 20P?

The 5821 is a wireless temperature sensor that can be combined with an external probe (470BP) for flood detection. So you can effectively get 2 sensors out of one (for example, you could monitor a basement to determine if the temp gets below 45 degrees and monitor for leaks in the basement with this single setup).

It can also be combined with a remote temperature probe (T280R) for fridge or freezer monitoring, which is how I'm using it (kids keep leaving the fridge door slightly ajar at night).

I think you have to choose between the fridge/freezer or flood probe - but either one works with the 5821s built-in ambient temperature sensor.

FYI, you typically have to buy the 5821 and the 470BP / T280R separately (I only mention this because the listings for the 5821 on many sites can be interpreted as including the flood or fridge probe, which is kinda misleading).