For several divers and dive guide I have met Con Dao is seen as the best dive site in Vietnam. Being away from the main land it helps to avoid some of the pollution caused by tourism and the incoming rivers along the coast.
There are currently three dive centres on the island: DIve!Dive!Dive!, Rainbow Divers and Con Dao Dive Center. The first two are based near the pier in Con Son town, while Con Dao Dive Center is in the centre of the town – near the market.
I only used Dive!Dive!Dive, and cannot comment on the other two.
Dive centre: Dive!Dive!Dive!
Dive!Dive!Dive! is located on the main road of Con Son Town, Nguyen Hue, only 2 minutes away from the pier. It is owned by American Larry and his partner. Both are lovely people who are happy to provide information beyond diving. They offer several packages, normal dives during the night or a special package, that includes 2 dives, diner on the boat while watching the sun set, and a night dive. Prices on Con Dao are higher than on the main land due to the limited numbers of tourists, and the costs to get supplies there.
They have a bespoke build diving boat – Quynh. It has no sun deck, but plenty of space on the main deck, and a spacious dive deck. It even has its own compressor to ensure there is always enough air available for a dive. WAter, fruits and cakes are provided throughout the day.