It seems to me that diving in Nha Trang, and Vietnam in general, has been unfairly labeled as "too easy" and/or "only beginner/refresher dives". I'm not exactly sure how this came about, but I am trying to help set the record straight.
Yes, it is true that a lot of the dives here are shallow (~18m). However, there are great dive sites around Nha Trang that extend to 40+ meters easily, such as Lighthouse and Electric Nose. These sites have fantastic topography, wide varieties of hard and soft coral, and rare/unique macro life with the opportunity to see pelagic life as well.
Hon Mun Island, where most of the diving takes place here in Nha Trang, is a Marine Park, and thus is protected from fishing and development. As a result, the coral has remained quite healthy, and there is a good amount of marine life diversity existing there. It is a popular destination for underwater photographers and admirers of macro life.
That being said, if you are new to diving, Nha Trang truly is an excellent place to get started because there are many dive sites here that typically have favorable conditions for training. However, that is hardly the only diving attribute that Nha Trang has to offer.