Spoke with Tom at Off the Wall. He was very helpful, called immediately after I sent my email and answered all my questions. We can schedule a boat dive with them, but I am a little nervous about the schedule. Planning to be off the cruise ship by 7:15 and on a dive boat at 7:45 seems like wishful thinking to me. Though he did say that they would pick us up at the port.
He recommended Sundivers as a good alternative and gave me a phone number for Olin. He said the depth would certainly be within our son's limit and it would be an easier dive (and a shore dive would be good for me because I tend to be seasick!) I love what I am reading about Turtle Reef, and the guy at the shop was very helpful. We think we would like to do the guided dive so we see all the best stuff out there. We could swim around on our own for less money, but we just wonder what we might miss.
So, now we have to figure out how to get there . . . it looks like we need a ride! 15-20 minute drive from port according to Google. What are the cabs like in Cayman? (I wouldn't venture a cab ride on Nassau, but they were actually pretty nice, and cheap, in Puerto Morelos, Mexico.)