A good idea when ditching in cold water is a possibility is to have an external bladder with a breathing tube which allows you to take a breath from the bladder rather than a lung full of water during the cold shock stage on immersion. A nose clip is usually part of this product.
They are great for reducing the panic when attemping to exit a submerged craft.
The problem is that if you're inside a surmerged and sinking aircraft, you don't want anything boyant attached to your person...the reason that life vests aren't inflated until you're outside the aircraft and clear of sinking debris. The vest, and any other boyancy, and trap you. and prevent your exit.