As I wrote in another thread just weeks ago:
I live in Vietnam for more than 5 years and dove in different places. Phu Quoc is by far one the worst place I have ever dove.
The visibility is bad due to the Mekong Delta.
The depth is ridiculous, 4m to 15/20m max, with an average around 7m.
There is very little fish activity and diving gets boring very quickly...
If you want somewhere with decent diving, just head to Nha Trang instead. The diving is not great but still much better than Phu Quoc.
I recommend Nha Trang Sailing Club Divers. Great service and price.
It is also possible to dive close to Da Nang/Hoi An (Cham islands) but from what I heard, it doesn't really worth the trip either.
Finally, there is another spot north of Nha Trang called whale island which is okay too. But again, don't expect to be impressed like when you go to Indonesia/Philippines.