I did the shark reef last week with DS11 and DS7. DS7 was scared and he has snorkeling experience in Hawaii. I held his hand all the way across, but he was enthralled right away with the sharks and never took his head out of the water. You really don't need to swim to enjoy the reef, but being comfortable in the water is probably important. It is very cold and deep. There is no where to stand. Several small children got scared and didn't try it. Since the water is salt water, it is more bouyant than regular pool water. None of us wore vests and had no problems floating. The fish were great. We saw leopard sharks, hammerhead sharks, and sting rays. Also saw the Dori fish from Nemo. I would have done it several more times, but the line was about 30 minutes long.
There is a lifeguard in the pool and will help when necessary