When I was spelunking we would often get wet. Florida caves are vertically challenged and tend to have flooded passages and sumps. Water temperature is about 68F. That probably doesn't scare any of you Yankees but at 68F and wet you can still suffer hypothermia.

Solution is to use a garbage bag with a hole cut for the head over the person sitting. Between their legs, careful with the fire, you place a candle, alcohol lamp or carbide light.

The heat of the small flame is trapped by the bag and the person warms up rapidly.

IMO the bags for any survival or emergency use should be 2mil, or better, cross-linked contractor bags.

These bags have loads of uses:
- Emergency rain or chemical protective poncho.

- Last resort clothes for when all the rest of your clothes are wet or contaminated. A lot more comfortable than standing naked in the cold wind. Also for modesty. If such things seem important.

- A good way to convert your pack and boots into a float for river crossings. Getting them across dry and you across safely.

- Slit up the middle they make a workable pack cover.

- A short-term bivy bag. maybe one for the legs and another, with a hole for the face, for the top. It won't breath so ventilate and modify as needed to prevent getting sweated up. I used one just for the legs and it added at least ten degree to the sleeping bag. When my legs got too warm or damp I just flapped the opening a bit. Got me through a very cold night.

- Stuff the foot end of your sleeping bag into one to keep it dry when the rain is blowing under your tarp.

- put a smooth rock or acorn in the bottom corners so you can tie on some light line and another set at the top and you can make a workable light-duty pack. The tougher the bag the more you can carry.

- I suppose it would make a makeshift body bag. Probably better than nothing.

lots of other uses. Limited only my their availability and your imagination. Good quality garbage bags, duct tape and a hank of light line can get you out of a lot of spots.