Here's another one that you're going to hope you never have to use, but you'll be so happy you know it if you do. The first thing to do is not panic. That's easier said than done. As your body is plunged into ice water, the first thing your nerves are going to want to do is panic. That's like saying "don't flow blood through your body." It's not something you can help. You can try to control it the best you can though.
I'm sure part of you think that you could easily do this, but once you're in it, it sucks. Your muscles won't work as well as they should, and you won't be able to breath. Those two things alone could kill me if they happened while I was sitting on the couch.