I agree that it may be best to go with the op affiliated with the cruise. We have gone out with DNS, DiveTech, and Ambassador when we've been on GC. All of them leave too early to do the morning two tank and the PM two tank that leaves at 1/1:30 (if they do one - most do a one tanker to the wreck or stingray city as noted) might be cutting it close for getting back to get to the boat. It is a long surface transfer to the North even from the West Bay dock so unless you hire a private charter, that would be incredibly difficult to fit in the time you have. I really like Big Tunnels and Lighthouse Reef on the west end so even though it wouldn't be the North Wall diving you want, it would still be very nice diving.