First f all, Get youself a very good accountant and bookeeper as well as a lawyer. Second, make sure that you have a staff that knows the RETAIL business and can SELL!!! Make sure that the people selling your stuff have a good PRODUCT KNOWLEDGE or have someone there who does. The most important thing, in my opinion, is to remember that the dive business is like any other retail business, and SELLING is the key. Be a good salesman or have someone/some people who know how to sell!. The classes are not where you make the money, its the selling of gear and rentals is where you make the money.
The dive business can have a great and friendly relationship with other local dive shops, but do not be fooled into the idea that they all work together and want the same for you as they do for themselves. They may want you to do well, but they want to do better than you and they would rather have your share of the customers than have to share them with you.