Supported temperature sensors - these appear in the list of found sensors.
The ELA BlueCoinT is an industrial BLE temperature sensor from ela innovation. They are not cheap (€31), but are waterproof (not hot tub proof though, sorry). It is sealed and had battery build in - non changable.
I purchased some of these direct from ELA, and they arrived and were announcing temperature via BLE.
It appears with the name as printed on it, usually somethiung like C T 802B03, though the app allows this to be changed.
However I ordered some from Amazon and they were not advertising. I had to download the app and activate via NFC - which was very tricky as the sensor is small, so has to be in just the right place by the top of my phone.
It seems there are a lot of very cheap sensors (£9), even with displays. Amazon sell the Mi Temperature and Humidity Monitor 2. However, it seems the price can be as low as £3 from AliExpress.
It does need a CR2032 battery. But the battery is replaceable. Not waterproof.
These sensors seem to require an app, and then to be activated. The app needs a login, I nearly did not add them at all. However, it seems someone has done some customer firmware and it is easy to reflash!
The instructions are here and pretty easy to follow. You go to a web page, select Connect and find the sensor (e.g. LYWSD03MMC), select Activate, then Custom Firmware, then Start flashing. Simple as that, takes a few minutes.
You can reconnect (it changes its name to start ATC_...) but then you have loads of options.
The default custom format advertisement is understood. The device will appear with its MAC address as its name.