On our tours, DD on one, and then DS on another - were totally petrified! But, the CMs are kind and wonderful at easing any concerns snorkelers might have. And by the end, each kid was sad the half-hour was over! (The CMs say they see that all the time!)
Since you're on the surface, you actually don't get near the fish (they usually keep away especially because they know you don't have any food), the sharks (they're a gentle type and stay far away on the bottom of the tank) or the turtles - you'll get warned about not getting close to them - not because they're vicious, but because they're a protected species and are NOT to be touched. If you have the chance to join in on the tour...maybe consider it. It is a wonderful experience! Good luck and have fun whatever you decide -