This reply is a little late, but for anyone who stumbles upon this thread in the future, it might be helpful. I am currently living and working in Nha Trang. The diving here is actually quite nice...healthy coral and stunning macro life. Hon Mun Island, where most of the diving is done here in Nha Trang, is a Marine Reserve. As backpacker pointed out in his experience write-up, most of the diving here is quite shallow (within 18 meters) and is not affected by currents or surge too often. It is an excellent place for doing your first open water course, and underwater photographers generally enjoy the great amount of macro life to be lensed here.
It's true that the low season tends to bring rain that can limit the visibility at times, but that is not always the case considering the topography of Hon Mun. For example, we have had two large storms come through this past week bringing strong winds and waves from the northeast, but the southern part of the island had a calm surface with 10-20m visibility underwater.
In addition, there are deeper dive sites here, as well. One in particular, Electric Nose, is one of the coolest dive sites I have been to in Asia. So, there are definitely more challenging dives than the ones you experienced here (I noticed you mentioned Moray Beach, which is a popular dive site here in Nha Trang given its location on the island).
As for dive-ops here, my opinion is somewhat biased, so I will refrain from mentioning any operators by name in the thread. However, be aware that not all the dive-ops here in Nha Trang have their customers' interests in mind, and that you generally get what you pay for in terms of level of service and safety. Feel free to PM me for further information (and perhaps a slight sales pitch
). I would be happy to answer any questions about diving and traveling in Vietnam.