I have been on some ships where Royal Caribbean actually does scuba training in their main pool, which is probably all of 5' deep. I think they were primarily doing the course work for their Discover Scuba excursions offered at several of the ports. But hey, if a person can maintain neutral buoyancy in 5' of water.....!
On most ships I think they do it in the normal pool as you said. On the Oasis class ships they do it in the Aqua Theater which is about 18ft deep max (Depth is adjustable by changing the floor height).
I am half surprised Royal Caribbean has not created a distinctive specialty for Cruise Ship Diving...