when we did this for the first time in 2000 my son was 5 and quite small for his age. He was not a great swimmer, but could tread water and propel himself somewhat. There is not test before you go and as far as i know, no height requirement.
My son had on the vest and the goggles, but couldn't get the hang of the snorkel. DH ended up tucking DS under his armpit and slowly "draging" DS across the reef. DS would hold his breath, stick his face under to see the fish and pop his head back out to grab a breath. He loved it and they went across this way 3 or 4 times.
- lori