A lesser known (but extremely effective) survival tactic is displayed here, by the Crazy Russian Hacker.

If you ever find yourself lodged in a body of water with nothing more than your clothes, consider stripping off your pants, tying them up, catching air in them and using them as a floatation device.

Here, the hacker shows us step by step how to do it.  He seems to really be enjoying the tutorial, as it goes on for many minutes more than necessary.

To recap, tie the pant legs together at the foot openings and capture air inside.  Keep the fabric wet so that it continues to hold the moisture (a dry pair of pants will lose air through the fabric) and continue to scoop more air inside as needed.  It can take you from a state of exhaustion to that of relative calm and it may actually save your life if you need to take an epic swim.