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I know this is late but it might help to clarify the issue some. My understanding is anyone fully certified but under 12 years of age, is limited to a maximum depth of 40 feet (and dive with a parent, guardian or dive professional as a buddy). The 2 Tank boat dives offered under contract by Don Foster's Dive Cayman, both dives exceed that depth limit (meaning the bottom is deeper than 40 feet). Hence it is pretty difficult for them to accommodate divers under 12 years of age. Also an "unwritten" (but well known) rule refrains them from taking "regular reservations" from divers wishing to go on the daily (non cruise ship) dive boat. Carnival will not hesitate to "end the contract" if some how they found out. The cruise line can not keep any diver from booking so they resort to these tactics to stop the operators.
However something to consider is that Don Foster's offers a daily 2 tank dive. It is usually no crowded (except may be during Christmas season), most times I find 8 to 10 divers. Many of them residents who dive often. In fact, they have gone out in many instances with just two divers. Very safe,flexible and friendly crew friendly. I think they are underestimated but have been in business for over 30 years.