Con Dao is probably the best place for diving, with the best corals and unique underwater life. The island of Con Dao is the biggest breeding ground for turtles in Vietnam. There are even dugongs spotted.
Whale Island is another famous diving spot, were even Jacques Cousteau did a lot of diving and research. The island lies for the coast of Nha Trang. The city is called the diving capital of Vietnam. There are a lot of dive schools here, but underwater life is less good. But in the right season it has good visibility and warm water; a good place to learn diving or train your diving skills.
The island of Phu Quoc is also one of the big diving destinations in Vietnam. Corals are not that deep, so it is also a nice place to do snorkeling. Cham Island in front of the coast of Hoi An is one of the only two marine parks in Vietnam. It is one of the better places for diving in Vietnam.