We are planning our first cruise, on the western Magic in May, and are looking at the various excursion. Has anyone been on the Atlantis Submarine excursion at either Grand Cayman or Cozumel? The idea of being able to ride in a submarine and see the underwater world really appeals (particularly since swimming is a problem!). Is one or the other of the two sites better...more gorgeous, better facilities, etc?) Should we book these well in advance or are we safe to wait until we check in for the cruise?