Palancar is the reef most people mean when they say they want to dive Cozumel, and it behaves less like a site than an address. The formation runs roughly three and a half miles along the island's southwest shelf, and operators have long since carved it into four named sections that dive like separate places: Gardens at the north end, then Horseshoe, then Caves, then Bricks furthest south. Published depths across the whole thing run from about fifty feet down past a hundred and ten. Which section the boat picks decides almost everything about your morning.
Gardens is the shallow end, and the one a shop will put a newer diver on: rounded coral heads separated by white sand channels, long bottom times, and enough light left at depth that the sponges still read red rather than grey. Horseshoe is the photograph everyone has seen, a crescent-shaped notch cut back into the wall and filled with coral pinnacles and barrel sponges wide enough to sit inside. Caves is the maze — tunnels, chimneys and cuts through the reef crest that you swim into and out of rather than past, which is why it rewards people who have their trim sorted out. Bricks is the deep one, the most southerly and the least forgiving, and it is where a shop sends experienced divers rather than the group off the morning cruise boat.
What links all four is the current. Cozumel is drift diving. You do not swim the reef; the water carries you north along it while the boat tracks your bubbles and collects you wherever you surface. That changes the etiquette more than the technique. Stay with the group, do not try to kick back to a coral head you have already passed, and sort your buoyancy before you are alongside the wall rather than on it.
Two rules here are law, not preference. Everyone entering Arrecifes de Cozumel National Park by boat pays a per-person daily park fee set in pesos. Gloves are generally prohibited on Cozumel dives, on the straightforward logic that a diver wearing them is more likely to grab the reef. Standing on coral is prohibited outright.
Snorkellers get Palancar too, but a different Palancar: boats work the shallow shelf above Gardens, not the wall.