lpjpane, my husband and I have always found that the cruise lines "plays it safe" with their excursion vendors they hire. They don't want any daredevil tactics or insurance scares. They certainly don't want any "accidents" happening. Excursions through the ship are pretty regulated and, if you don't mind me saying, boring. As long as your husband understands that excursion guides will closely watch every diver, especially near and around the protected reefs.
Time is another consideration when making your plans. You are limited to time spent in the water. A 3 1/2 hour excursion technically means you're diving for approximately 1 hour to 1.5 hours of that total time(the time seems to fly by too.) The guides allow for travel to and from the excursion destination and for time spent waiting. That's their buffer.
If he is an experienced diver, who is used to diving with a buddy off a boat somewhere in the middle of the ocean, don't expect that same scenario when you book an excursion through the ship. Excursions booked through the ship are pretty much... methodical.
You can do some research and find your own private diving guides. Just keep in mind, when you book privately, the ship is not responsible for whatever may happen. Hope you find what you're looking for. Have fun.