All three on the tank are TO-92-packaged “raw” DS18B20 sensors: the top one is wedged under the insulation; the other two are cable-tied to the pipes right next to the tank.
I have a box with an ESP8266 (Wemos D1 mini) and a couple of relays running as a controller for the gas central heating and gas hot water (Y-plan system) — these three sensors as well as the one for loft cold water (waterproof DS18B20) and loft ambient air (another TO-92) are connected to that and reporting to HA.
The loft ones are to protect against freezing: if the temps get too low if we’re away in winter with the heating low, it can run the hot water to soak some heat upwards and warm the cold water tank. No idea how likely that is, but I can find out now I have it instrumented.
