And all very shallow. In very clear water which is to say the best of conditions, like we have in Cozumel's water red light penetrates no farther than three feet, yellow no deeper than nine. After that, it's just blue and your brain's ability to fake it. So shallow very shallow reef is best for snorkeling for color. And color is, after all, one of the many grandiose miracles of the reef lost to most visitors to it.
Trouble is, most shallow reef has been kicked to death by visitors.
Witness the west side of Cozumel. The best efforts of the local divemasters have preserved the heavily traveled reefs in rough good condition. One certainly gets a feeling for the majesty of what lives there. But they've clearly suffered from a lot of traffic. The beaches in the north hotel zone and the south, for that matter once had abundant offshore coral heads and inshore cliffs alive with sponges, corals, urchins, and fish of rainbow hues.