Also depends on how confident they are in the water and if they can snorkle which is quite tricky when you start doing it. Personally I would wait till the children were older and stood a better chance of remembering all about the day when they are older as it is an expensive day.
My first visit to Orlando was when my sons were 7 and 5 and they can just remember bits of the holiday. My Mam was a bit put out that they couldn't really remember much about the holiday as we went with my parents and sister and it was special from that point of view.