Empty PET bottles can be very helpful. Here's one amazing example! Using them with t-shirts you can make life jackets. [From "How to Craft Safety"]
Materials and equipment: T-shirt, ten 500ml PET bottles, string (about 1.5 meters length)
How to craft:
1. Place PET bottles inside the shirt, and secure into place by twisting on the cap from outside the shirt. (5 bottles in front of shirt, 5 bottles in back)
2. Tie around waist and fix into place.
Memo: This is merely a temporary substitute for an actual life jacket.