.2 deck will help take some of the edge off bombs
.2 belt will reduce damage from torps
bulge again, more protection from torps
bulkheads will reduce the amount of speed you lose from heavier damage (RE: instead of losing speed at 60% damage, you might not lose speed until you're 30-50% damage, depending on the amount used).
just remember not to armour up too much that you're a sloth on water out there and an easy target. from bombers, subs, as well as other bbs