In Bonaire, the bottom is sandy with sprinkle of rocks and coral to about 50 yards? At that point, solid reef begins around 15-20ft in depth and increase depth gently until about 35ft. After that it becomes a reef wall going down to depths of 100-150ft. I found an abundance of life from 20-50ft. Below 50ft was more of the same but with less color due to depth. Above 20ft it was less abundant but similar life. In 5-10 ft of water I saw barracuda, parrotfish, anglefish, urchins and other typical tropical life. The only drawback to snorkling I would note is that the beaches are coral and too sharp to casually walk barefoot. Booties or some sort of shoe would be recommended. If life at a site is poor for snorkling, there are plenty of others neary to try.
I snorkled in Cozumel and found things worth seeing from shore entry but not enough to suggest going there for that purpose. If you go to Coz/Cancun/etc to snorkle, I suggest a trip to xel-ha water park. It will be crowded. A boat trip will be ok to snorkle but you will not be close to the life. You will be able to swim to the top of the reef but only to swim by. I don't reccomend it as a snorkling destination.
I can't speak of others until I go. I've heard Turks may be good.