It's a Tank (88cm x50cm sized, 15cm DEEP) used as a foot bath
If the tank is a metal (or at least heat conductive one you can add the sensor on the outside. The temperature gradient through most metals is very low, so the temperature measured outside the metal is only 0.1C or less lower than the inner temperature.
be aware not to put the sensor near the hot/cold water inlet as you get biased readings ==> advantage of measuring outside the tank.
However measuring outside the tank takes a bit more time than in the water, so your SW should adapt for that.