I am going to TL for the first time on my next trip, so I can't speak from first hand experience, but according to the Passporter, you can scuba in the Shark Reef area. From page 173 of the 2004 Passporter:
"Get your first taste of scuba in a special section of the Shark Reef. Cost is $20 for the first 30 min. and $15 for the rest of your party or for repeat dives. Pay at the booth near the dive shop. Brief lessons are provided, and parents can attend their kids without renting."
While we're on the topic of TL, I have a question I posed but no one ever answered, so I'll try again. How does Gang Plank Falls work if I am a solo traveler? Will I be paired with another group since the raft holds 3-5 people, or am I just not able to ride it since I don't have 2 people to bring with me?