@ABCsOfLife for the amount of water inside the barrel, the fire won't change the temperature that quick. The barrel would only be a bit hotter than the water, because the water would sap out the heat first.
@ABCsOfLife
Adding onto @narra 's explanation;
There is also a wooden skid inside the drum to step on so your feet wont cook.
I think its explained in arakawa under the bridge ep2 or 3.
Thanks for the upload - and a nice high res scan too. Female Aku is a bit more obvious at this res. That eye belongs to a pretty girl or an S class trap.