I actually used this company on a trip to Nassau a few years ago (not a cruise).
It seemed like a respectable company. They taught us how to use the masks and were very knowledgable about the landmarks we passed.
We didn't go to a shipwreck, however that would be pretty neat I would think. We did do the shark dive and what happened was that they have a place that they always go to. The reef sharks seem to know that they will be fed in this one spot and swim along the bottom. You jump out, each of you holding onto a rope and quietly watch them on the bottom. After everyone returns to the boat, the people from Stuarts Cove feed the shark and soon it's a frenzy on the surface. One of the sharks jumped on the platform on the back of the boat and the person had to jump out of the way very quickly. I have a picture of this.
I saw this company in the news a few months later. Evidently someone was bitten by a shark, but I don't remember if it was an employee or a customer.
Just use your best judgement and it is a wonderful time.