I do not know anything about starting a business in Aruba, but I do have a couple suggestions, as a fellow diver and recent college grad.

1. Try to find a dive club with your school, and participate in group trips to see what you like and dislike about other dive shops (your soon-to-be competitors).
2. Go as high as you can in the PADI schools. As a diver who goes on dives and/or rents equipment with scuba stores, I want an owner who is a divemaster and/or an instructor.
3. I know you are an EMT, but make sure you have taken the scuba first aid classes. You will probably end up saving some diver who almost gets himself the bends or narcosis.
4. Some people want to dive with Nitrox, so you'll need a certification in that, too. Though I would imagine that is not nearly as important as 1-3.

Good luck and let us know how it goes!

Aruba History:
Lou: Visited on a cruise in 2005
Lou&Nay: Will visit on our honeymoon 9/2008