We did the shark reef and I would estimate that it was about 10-12 feet deep, maybe a little less. The sharks are small and mainly stay to the bottom. When you start out, they basically tell you to move along slowly, but keep up the pace and go straight to the other end. It is not a free-for-all and you can't swim around in circles, opposite direction, etc... You can take your time in enjoying the underwater view but have to keep moving.
We had a mishap with the whole incident, but would probably do again. My 9 year old son (never had snorkeled, but swims) had a mask that the seal wasn't good, so we all started out, my husband first, my son next, and me last. Well I popped my head out a couple times to see if son was alright and after last look, heard noise in water, popped head out again and my son was panicking in the water. I had to yell to husband and Cast Member to get to him immediately because I was at least 15-20 ft ahead of him and he started going in the opposite direction. My motherly instict was to swim as fast I could to get him, but one of the CM's yelled out "Don't flail your arms around so much, swim gently across". I soon found out later that was because I could aggrevate the sharks! And I wasn't doing anything except normal breast stroke to get my son! We had a good laugh after our trip, but my son isn't quite sure about doing it again. I'm sure if he was more experienced with a snorkel and mask he'd be fine.
Good luck - it really is quite neat. Just remember to swim SLOWLY!