I have been several times to SAI......the wife and son lives there.
The diving is spectacular…the best I have ever done with exception of Old Providence Island just to the north.
I have dove with all the DO on SAI, all are good, an I have been happy them and the experience they have shown me, you should be happy with any one you choose.
The best dive on the island is by far The Blue Wall, an absolute must. Being on the windward side of the island it is not always accessible. On the leeward side of the island are most of the other sites. There is a ledge along this side varying from a few feet drop to 10 feet drop, in 30 to 60 ft of water, lots of coral, sponges and animals. There is also a cool shipwreck on the sandy bottom in about 35 feet with the mast sticking out of the water in low tides which is great for fotos. Numerous patch reefs 10 to 25 feet on the north part of the island great for macro fotos and loads of stingray.
Acquarium and Coco are both nice resorts I have heard although I have never stayed at ether I have visited both.
The Acquariun is in the town on the sea so there is plenty to do shop, restraints, bars, ect. within walking distance.
Ccocplum is in or near San Luis on the sea, on the Sothern part of the island and away from the town center, it is more like a small village with a few restaurants around.
I would like to hear what you have to say about them after your trip.
Let me know if you have any other questions.
Ciao
Tad
ps:my DM friend does frelance dive guides if you are interested in getting personalized service for your dive trips, at DO rates