If you have to ask about how to get a Dive Shop up and running you are likely not in the position to start as of yet.
If you are wealthy, and don't need to actually make money in the industry, then you will likely do very well! However if that was the case, I doubt you would be posting.
Even buying a small business that is already established, unless you can afford to pay for everything out of pocket, you are going to need to do a HECK of a lot of research. You will need a business plan to start, and will need to obtain financing. That is actually the easy part. The hardest part is making decisions about location, wholesale connections, what lines to carry, how to handle payroll, personal, taxes, finances, liability, insurance, customer policies, mastercard/visa, bad checks, marketing, advertising, on and on.
It's a business, and if you have never had any experience running a small business, you also may need to pay someone to help set you up, and then hope to god you can actually pay the bills before you run out of $$$, and go bankrupt.
There is a big reason that so many Dive Shops go out of business, and it has little to do with the internet, or local ecnonmy, and more to do with the fact that you have to really know what you're doing to grow a business from the ground up.
The few business plans I've seen generally assume that the business will not profit within the first year, and make allowances for little profit even after two years.
