It's hard to make a bad choice of dive op; virtually all on the island are very good, and after all, the diving is the thing. There are differences between Coz ops in terms of features and amenities, though, and the price varies accordingly. Some transport divers only in small groups while others have larger boats. Some are "valet" operations, which do everything for you, including storing and cleaning your gear between dives, while others leave that up to you. Some will take you to a beach resort between dives while others SIT on a remote pier or just cruise slowly to the next site, and the ops based at the southern resorts will frequently just go back to "home base" between dives. Most use 80 cu ft tanks while a few provide larger ones. Most of the small boat ops decide their destinations by consensus of the divers aboard (with the DM and captain having the final say), while the larger boats usually follow a set schedule. Some will put experienced divers on different boats from newbies, while with others it is the luck of the draw. There are "no frills" ops and pampering ops and everything in between. What sort of experience were you looking for?
