Looking for any updates as this topic has been discussed previously.
Need to monitor the temperature of my hottub. Being in a cold climate the risks of frozen pipes.
Wish List
Unable to find a device and thinking of hacking a Sonoff SNZB-02 or a Smartthing Multipurpose sensor and solder the probe directly to the sensor.
There has been some recent discussion on this, but so far no reports back from people who were trying different models:
Meanwhile, what country are you in? The device selection does vary.
Northern Canada
You dont have to hack anything Sonoff do a th16 (or th10 if your not switching more than 10a)
But neither ate battery operated. Surely if your monitoring a hottub you’ll have a supply you can connect too. I do!
My th16 is within the body of the hottub and connected to the 240v supply. I then have the temp probe coming out through the drink bottle ‘ice bucket’ and into the tub.
The probes are sealed so no issues with chlorine.
Very very similar to your pic but terminated with a 2.5mm jack plug so it’s plug and play
Does the TH10 communicate temperature or binary contact to ST.
I currently use a Smartthings Multipurpose contact sensor vacuum sealed twice with a pool noodle.
The Good: No problems with connection, battery corrosion, Its a cheep fix, and I get real time monitoring.
The Bad: Battery life is greatly shortened(about 2 Months) I don’t know why yet because I have the same setup in two aquarium canister filters. Both of those are still going after 5 months.
It reports both the temperature and condition of its relay switch.
Batteries can and do outgas.
It’s not safe to vacuum seal or place any battery operated device in an airtight container unless specifically designed for that purpose.
Yes I know but there aren’t a lot of options out there. Figure there worst that can happen is the sensor fails. I do have a couple of the older fibaro contact sensors with these probes for my freezer and they work great. I am still looking for a contact sensor replacement for these probes as I still have there.
Oh, it can get worse than that. 
If This is a swimming pool, toxic materials from a leaking battery can get into somebody’s eyes. And there’s also a potential galvanic corrosion fire hazard.
Best advice is just not to use any sensors in a pool which are not designed for pools.
Thanks I will re think about it