There are other types of antiship missiles that dive from above. Battleship deck armor is considerably thinner than its belt armor. The 17-inch figure is the maximum thickness in certain limited areas. It's much less almost everywhere else.
The battleship thread comes up periodically on the sci.military.naval Usenet newsgroup. It's dismissed out of hand by the people there...many of whom have actual expertise in the field: active and former naval personnel, marine engineers, etc.
The Iowas vertical protection was designed to resist aerial bombs. The ships have armored decks for the first three decks totalling 12" of vertical protection not including a fourth splinter deck. Acoording to Summerall who has written one of the best researched books on the Iowa Class it would take an armor piercing bomb dropped from an altitude of 10,500 ft to penetrate the ship's vertical protection. No current missile which pops-up before it dives on its target goes to that altitude nor does it have the warhead weight or explosive power to do as you say. The Missouri was hit by a kamikaze during the battle of Okinawa which was carrying two 500 lb bombs which only caused superficial damage to the superstructure.