Advanced divers heading for the Cayman Islands are finding that it's easier than ever to enrich their diving experience by simply placing an order for their gas of choice: enriched air nitrox. This ''designer air'' is now readily available on all three islands.Dive professionals on-island, along with savvy visiting divers, are avid consumers of nitrox, and almost every dive boat has at least a couple of bottles with the prominent green and yellow bands. Operators charge extra for the gas, but proponents say it's worth it to reduce diver fatigue and nitrogen buildup.Of course, nitrox can increase bottom times as well, but few divers seem to be using it for that purpose. At Grand Cayman's Dive Tech, the first shop to pump nitrox, Nancy Romanica says that ''most nitrox divers are diving on air tables. They tend to be older, and they just want to feel better.'' Nancy's analysis is echoed in shops from Little Cayman's Reef Divers to Cayman Brac's Dive Tiara, both of which pump their own nitrox.On Grand Cayman, Aquanauts Diving also pumps nitrox, as does Pure Air, the third supplier. Little Cayman and Grand Cayman's North and East Ends are getting the greatest demand. At East End's Ocean Frontiers, Maurice Fitzgerald says, ''Nitrox is used more here than anywhere else on the island. About 20 percent of our divers are using it.''In classrooms throughout the islands, students are learning about partial pressures and atmospheres absolute. Dive Tech teaches several classes per week and also offers advanced nitrox, technical nitrox and even trimix. The mixture used here is almost exclusively EAN 32 - 32 percent oxygen (normal breathing air is 21 percent). Richer mixtures are possible, but as the oxygen percentages increase, the diving depths must decrease to avoid oxygen toxicity.For years, the Watersports Operators Association (CIWOA) frowned upon nitrox use. With so many dive sites that plummet hundreds, if not thousands of feet into the beyond, it's really not surprising that dive operators were at first hesitant to embrace an air mixture designed for medium depths, and certainly not beyond 130 feet.Ron Kipp, owner of Bob Soto's Diving and past president of CIWOA, admits he was probably the most ardent opponent of nitrox use in the Caymans. ''What can I say? I was wrong,'' he now admits. ''Year after year, people were using nitrox without incident, and I had to face reality. Nitrox diving is a form of advanced diving, and I think people who use it are very well trained,'' says Kipp.Says Lachlan McKenzie of Pirate's Point: ''Nitrox is becoming more popular on Little Cayman, and it's going to become bigger and bigger as more people learn it is not complicated to use - that it will increase their dive times or give them a safety edge.'' Not to mention that extra measure of comfort as they buckle up for the flight home. For more information...