Nha Trang has 10 or so dive shops, so this is a 'buyer-friendly' market! Students looking for certification will find that the value for money here is superb, with PADI open-water courses available with full equipment rental. A two-dive boat trip with equipment rental and snacks can be had for a very reasonable sum. As with everywhere in Asia, check what you are getting for your money and make sure you speak to the Instructor who'll be teaching you, not just a 'salesman'. Diving, and especially training, should not be chosen by price alone.