We just go back from a week in GC, staying out on the East end of the island. Although you will probably want to find someone on the West end (where you'll be staying) for most of your diving, I'd highly recommend that you consider a dive or two with Ocean Frontiers (OF) on the East End - simply one of the best dive ops I ever used. The trip from where you will be staying on the West end to the OF dive shop on the East end for a morning 2 tank dive (departs at 8:30) would probably take you 60 minutes in "rush hour" traffic, but it would be worth it for the great wall dives that they have out on the East end.
Since they also run afternoon dives you might consider this as well, and there would be less traffic.