As usual, good advice from all.
Cruising permits are issued for 1 year and may be renewed 2 times. Renewal for my Hunter 30 was $500/year. At the end of the 3 year period you must either remove your boat from the Bahamas or pay the import duty. If you leave the Bahamas you can return, buy another cruising permit, and the clock starts all over again.
I've kept my boat in the Abacos since 2004, flying back as often as possible. Most of my time was spent at Sea Spray Marina on Elbow Cay. They took very good care of the boat and were great people to deal with. Unfortunately, dockage prices have gone through the roof. Off season rates (Sept-Feb) now run around $.65/ft/day, but jump to $.90/ft/day during peak season.
If you plan to fly back and forth as I do, you may find it more economical to dry store your boat at Abaco Yacht Services on Green Turtle Cay or rent a mooring ball in Man-o-War or Hopetown. There may also be some long-term moorings available on GTC.
If you choose to keep your boat in the GTC/Treasure Cay area, plan to fly in and out of Treasure Cay Airport. Otherwise, you'll have to take a $70 to and from Marsh Harbour. Several airlines provide daily service from the States.
As far as diving and snorkeling go, you've got lots of great choices- Brendal's on GTC, Treasure Divers at TC, Dive Guana, Froggies in Hopetown, and Dive Abaco in MH. There is also a new dive operator at Boat Harbour Marina in MH.
One last thing, if time permits, head south down to Little Harbour. Pete's Pub is right on the water, next to a working foundry and gift shop. You can also snorkel Sand Cay Underwater Park along the way.
For more Abaco information Google "abaco forum" and you can access the Forum of Message Board websites.