First of all, find out what dealerships are available to you and what their initial order requirements are.
Manufacturers, not surprisingly, like to protect their existing client base. Supporting you might lose them long-standing retailers; so find out if your proposed location is one you'll receive manufacturer support in i.e. there's no point in applying for a dealership in Acme Regulators two blocks from Billybob's Dive Emporium if Billybob has been selling Acme Regulators since 1963. If Acme haven't got any retailers within, say, 50 miles you'll stand a better chance.
Be wary of initial stock orders. Some manufacturers will require surprisingly high minimums. You may well be able to afford them but you might end up, in order to stock a range of brands, doubling or even tripling on stock lines. That ties up a lot of cash and floor space. Contrarily, it would be very courageous to go with one manufacturer and stock only their products. For one thing, not every manufacturer is great at making everything, there are inevitably items in their inventory that suck and won't sell. For another, divers tend to be educated shoppers; think how rare it is to see a diver wearing a single brand of everything. If you stock only one brand then you're excluding all the customers (and there will be many) who don't rate your particular brand.
Also, be aware that you'll probably be invoiced pro forma, at least initially, very few distributors will give you a credit account until you've built up a trading history with them. That will impact your cash-flow for Year 1.
The final point that springs to mind is don't underestimate the cost of your air/gas infrastructure.
Best of luck.