Hi,
Went diving both in Nha Trang in 95 and 5 years ago on a location called Whale island (Hon Ong), 70km further up north on the coast.
Though Nha Trang was a bit sloppy on the viz and fish life, I remember the hard corals were fine at some places.
Whale island is another story, it is based on a lake like lagoon, absolutely idyllic place reminding a Maldives resort, with sometimes great viz. the resort is french/vietnamese owned, the dive shop on premises is one of Rainbow Divers'. The dive guides are incredible they totalize some 5000 dives in the area, be prepared for all sorts of critters like nudis, ghostpipefish, scorpionfish, etc. Strangely enough the nudis in this area look like they were bred on steroids, twice the normal size at least... They also cater blue water coral dives, their "three kings" and "belfry" spots are beutifully carpeted with soft corals... not Fiji size though but still beautiful enough for the area.
they also claim they sometimes have whale sharks visits but I didn't see any...
Dive season is from march to september, Whale island is a welcomed break from a trip in the Mekong, en route to Hoi An or Ha Noi...
Whale island compared to... I would say the regular sites of Koh Tao but with more interesting critters and better guides.
Other dive centers in Nha Trang : there are a few, mainly french owned like Coco dive, or Orca -originally Vnese and the longest standing-. Some offer crepes at the end of the dive, others don't...
OP question is about tek diving : I had been told there is a korean owned dive center in Mui Ne which caters for deep wreck dives and trimix. dunno much more...
cheers
Luko