My own pet theory is because Amon/Noahtok is a master water bender. You'd figure, he'd have to be, since he was also a blood-bender. We have seen water benders heal. We have sen water benders shield themselves with water. We have also seen water benders make themselves little impromptu submarines made of water (bubbles) and submerge. The writers may, or may not, go with it, but I think there's a chance he could have survived. After a point in training, especially under battle conditions, some defensive moves become reflex actions.
And I separated the words a bit, because my spellchecker is having a fit. razz