Help me find a temperature-actuated switch

Here's the problem:

Freeze protection for fresh water tanks exposed to outside temperatures. I can and will do my best to insulate them, but want a backup/failsafe in case that isn't enough. 

I want to wire a circuit that sends power to a servo-operated valve based on the temperature in the tank. Thermostatic switch closes when water in the tank falls below 35*F, servo opens the valve, circulates water through the hot water heater back to the tank to bring the temp up. Switch opens above 45*F.

So what I think I want is a submersible, closes-on-fall/opens-on-rise, 12v switch, so far as I can tell

Is that what I want, or just what I think I want? Where would I find such a switch?

Something like this? https://www.amazon.com/dp/B004LYO3ZO/ref=dp_cerb_1

Or what about this: https://www.amazon.com/Inkbird-Combination-Temperature-Controller-Humidistat/dp/B072JWF69R/ref=sr_1_6?keywords=submersible+temperature+sensor&qid=1574188304&sr=8-6

Or this one's cheaper: https://www.amazon.com/Thermostat-Inkbird-Temperature-Controller-Pre-Wired/dp/B07DN9K7PH/ref=sr_1_7?keywords=submersible+temperature+sensor&qid=1574188304&sr=8-7

Build your own setup with this: https://www.amazon.com/Electronic-Thermostat-Controller-DROK-Temperature/dp/B07GQPT9VG/ref=sr_1_15?keywords=submersible+temperature+sensor&qid=1574188304&sr=8-15

In reply to wae :

I think #1 is closest to what I have in mind, but I didnt see anything in that listing that says "yeah go ahead and let it live underwater" except that its "sealed". Other than that, I'd just have to figure out how to mount it through the side of the tank without sacrificing water tight-ness.

https://www.ebay.com/itm/NEW-12V-24V-110V-220V-STC-1000-Digital-Temperature-Controller-Thermostat-w-NTC-K/222684154843

I have 4 of those at my house. one for the kegerator, one for the fermenter, and 2 for whatever randomness needs a temperature controlled relay.