Again Elbert has the facts. But let me ask you a question: Why not just dive the reef? This would be much safer, cost less and give you the experience you all need. The reef is full of great dives and even those of us who live here never tire of them. The very best first (check-out) dive is Hol Chan. Hook up with one of the dive shops on the island; Elbert is one of many and a good one. We each have our favorites. Save the longer, more expensive dives for when you have more experience - like buoyancy control.
You will not be disappointed here.