We have done several Discover Scuba with SailCaribbean Divers at Cooper Island.
It works out about as good as it can! The Discover Scuba divers do the basics in the shallows just off the beach at Cooper. If there are Certified divers on the boat, they go for the 1st of a two tank dive, and return to the dock to pick up the DC divers, and then they all go for the second, shallow dive. SailCaribbean took really good care of my sister-in-law when she was having problems cleaning her ears.
I would make reservations in advance for Discover Scuba. You can sometimes slide right in if you are certified, but generally they need to know in advance for the introductory class.