My 14-year-old son and I had a private tour with Damon, who is an excellent guide. We saw everything we were hoping to see, and more... 4 or 5 green turtles, stingrays, porcupine fish, trumpet fish, even a dog that had swum out to greet us from a yacht moored nearby!
The scooters are great fun and easy to learn. You do go quite a long way out, but the scooters will give less confident swimmers a feeling of security. You are also given a life jacket and it is up to you how much to inflate it.
The Pillars of Hercules are stunning... but be careful you don't get too close to the rocks ;-)