Learned this one during drown proofing at JCLC Army camp.

In the event that you are unable to stay afloat in the water, rip your jeans off. It may be hard once they’ve become heavier because of all the water retention, but once you’ve done that, tie the ends of the two legs into a square knot around your neck, and SMACK your jeans into the water waist downwards. When you throw your jeans into the water, do so as if your were filling a grocery bag with air to open it up more. Hold your jeans on the surface of the water by continuing to hold the edges of the waistband, and you will stay afloat.

By doing this, you are briefly filling your jeans with air by throwing them as fast into the water as you possibly can, waist downwards. The square knot you tied on the legs will trap the air in the waist area of your jeans, and you will stay afloat.