I have found some rather nice little BLE temperature sensors with 5 year life, and waterproof.
Obviously the first thing to do it put one in the hot tub :-)
Of course I need something to receive this, and these are actually nice and send a proper temperature reading in the BLE advertisement every few seconds. So time to make some code for ESP32.
The problem is water and radio to not work well, so I actually need to put it close to the hot tub.
So this is the ESP32 gateway - it is one of the smallest boards I have made, and designed to go directly in to a USB socket.
And now I have a garden BLE/WiFi gateway :-)