Ok,
I just returned from the Cay Sal Bank trip on the Nekton Rorqual less than 24 hours ago. Here are my thoughts on your question...
Cay Sal is NOT difficult diving by any stretch of the imagination. The crew of the Nekton have a safety system in place for diving that is very good. What your daughter is getting into is probably one of the better ways to get experience. Now let me explain...
The Nekton boat leaves out of Ft. Lauderdale and dives a very shallow reef system the next morning...its meant to shake out the cobwebs for those that have them, and allows the crew to see who is comfortable with diving and which divers might need some closer watching. Your daughter and her father should probably say they have zip for experience when they arrive, but the crew will figure it out very quickly so don't worry too much abou that.
They then move to several blue holes that yes...do start around 60 feet, but in reality you spend VERY little time on the wall itself unless you have a reason to be there, and most people stay around the rim...these are well within the bounds of the OW certifications. There is a tremendous amount to see there...from eels, to sharks to rays to coral. I am not sure why the other poster didn't see much, but we had a tremendous amount of life on the holes.
The Nekton boats were designed for diving. Here is how it works.
You get your briefing on the dive site and then go down to the dive deck. There are 2 dive masters on the platform helping the divers at all times. There is a motorized skiff floating behind the boat whenever the dive deck is open in case they need to give someone a tow back to the boat. In addition to the dive deck, there is at least one dedicated bubble watcher up on the sun deck. Their sole job is to watch to make sure no one is getting too far away from the boat. The best part of the Nekton boats is that you can ALWAYS ask for a divemaster to go on the dive with you. They practically BEG you to take them with you because they get to DIVE! Your family should ask for one...its NOT a burden on them. Trust me.
They offer other certification classes on the boat, but since there is very little experience with diving, I would suggest they just ask for a DM to accompany them on their dives.
When the dive deck closes and the boat goes to move, there are several checks to ensure no one is left behind. I never heard the 2 DMs on the platform NOT know exactly who was in the water. In addition to them, there is a large sighout board on the way down the stairs that you sign out on with your name and time out of the water. You sign back on board when you get back and put your max depth down also so that they can keep track of what you are doing. Our captain walked to the back of the boat prior to starting up the engines, and verified that every tank was back in the racks below. Its a very simple check to verify that everyone is back on the boat.
The whole concept of scuba may sound a bit dangerous, but rest assured that on the Nekton they will be in good hands.
BTW...our boat had to young teenager girls on it. One was the daughter of one of the Nekton boat captains who was overseeing the training of some of the newer crew...the other young lady was born in Hong Kong. Were either of them yours? I know for a fact, they had a blast and got to experience and see stuff that most people cannot even imagine doing even when they watch it on TV.
Hope this helps alleviate some of your concerns...If you have ANY questions, just ask here or PM me. I just got off the boat and would be more than willing to answer them for you.