A Hot Tub

This a hot tub you could use at the survival time

Inflatable Pool

Aluminium Foil

First cover the inner portion of pool with aluminium foil just stick it .

Let it Dry

Extra!!!

Aluminium will reflect the sunlight and will heat the water so the water will become hot

Fill the water and the aluminium will do its work

If you want use this at night you could just dig a big hole burn a fire and put the pool over that hole but dig that deep that there should be 20cm distance between pool

The hole should be like the image i added

And then put the tub on it