I'm not military, so I'll def let the military helo guys give the answers, but we were told specifically to not wait until the water reached the bottom of our face. We were told to take our final breath as the water came up to our chest, because when it hits your face, it's too late. I took all my breaths by mid chest and it was true, by the time I finished breathing in, my face was submerged.
As others have said, muscle memory is key. Know your exit, know how to operate your exit, and know what stands in your way between you and your exit. Also, NEVER unbuckle until the craft has completely stopped moving. When I dunked the first time, I unbuckled out of panic, and ended up floating out of the seat and tangling in my seat belt as the MET was still rolling. Thankfully the safety divers got me out. As bad as it may sound, if someone is in your way and they aren't responding, or panicking in the way of your exit, do your best to move them aside and take over. When I dunked it really woke me up, knowing that the only thing that is going to save me, is me. There isn't time like in the movies to round everyone up and get them out, its you and you only.
Here's a vid of the group we were with dunking. This was just before I got into the pool...
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