I see you have been to Coz, so I will just give you the comparison to the Sister Islands. I dove all three in January and February this year. Coz and Little Cayman are probably the top two wall-diving destinations in the Caribbean, with the Brac right behind. Little Cayman's walls start shallow, in as little as 20-25 ft. There really is no drift diving like Cozumel. There is generally little current at Little Cayman or Cayman Brac. The boats typically moor to a buoy and the divemasters declare the pool open. Plan you dive yourself. Guides are not required. The crew will usually give a good dive briefing, drawing a map of the site on a whiteboard and suggesting a route. You can dive where you want, just navigate your way back to the boat and watch your computer. The water is incredibly clear and viz is spectacular. Corals and other marine life are very healthy at both of the Sisters, probably a little better at Little Cayman. The walls are on the north sides of both islands, with lower profile reefs and spur-and-groove formations on the south sides. These make good third dives and night dives. There is more good shallow diving at the Sister Islands than Cozumel, particularly since the hurricanes. We had a good dive at Palancar Caves this year, but our divemaster told us many of the shallower sites had suffered. If you like Coz, I think you are going to love the Sister Islands.
