A tub will work, sure, as long as it's pulled from a large body of water that hasn't warmed up with the air. Hopping in the local lake is an obvious choice.
But in a detached demographic sense: effectively everyone lives in a city in the modern world, and while their housing might not be air conditioned most of the major buildings are. In an emergency like this everyone would just crowd into office buildings and shopping malls.
That's not to say this wouldn't be horrifying, but it's not the kind of existential social crisis people tend to imagine. Cooling ourselves down in a heat wave, in the modern world, is a solved problem at least as far as keeping ourselves alive goes.
But in a detached demographic sense: effectively everyone lives in a city in the modern world, and while their housing might not be air conditioned most of the major buildings are. In an emergency like this everyone would just crowd into office buildings and shopping malls.
That's not to say this wouldn't be horrifying, but it's not the kind of existential social crisis people tend to imagine. Cooling ourselves down in a heat wave, in the modern world, is a solved problem at least as far as keeping ourselves alive goes.