Thanks. Good info., and useful. The old saying 'One man's ceiling is another man's floor' applies. A rocky bottom and iron shore shoreline may make for better snorkeling, but a little kid (like our 2nd grader) may be better served with a 'wade friendly' sloping sandy beach.
This has been brought up in past threads, and the one research study I saw on the subject was criticized for simulating what would be a very 'unchallenging' dive experience.
In other words, if it's the study I suspect it is, it did not re-create the kind of experience an instructor teaching a bunch of students in a day, or a shore diver diving 5 times/day in Bonaire or boat diver doing 4 dives/day in Cozumel would get.
Some people cry 'placebo' or 'anecdotal,' but when enough independent people with real world experience share the same anecdote, at some point that becomes evidence. A lot of what's true in the world hasn't yet been demonstrated to meet the rigorous standards of a scientific research study in a peer-reviewed journal.