Diving on the South end is way better than the North. We added 2 days onto to our trip to try it out a few years back and were disappointed. Our worst dive on the southern end was as good as any we3 did on the North. Haven't gone North since. The dive shop near the medical school up North was very friendly and accommodating but they simply need a better product.
When staying at Castle Comfort we never dove with the cruise ship boat. That is a separate boat.
Some of the best diving is on the outer side of Scott's Head but not always available because of currents. If your group is experienced enough get them to take you out there.
There are only 3 or 4 sites outside,Condos,Swiss Cheese and a couple others. They were very nice sites as the currents feed the corals out there. But really not for novices if the current get going.
Don't expect a lot of big fish as Dominica is more coral,critter and macro land. Lots of colorful crinoids and corals and plenty of uncommon stuff,electric rays,flying gunards,bat fish ...... Never been there (and go every year) and come home without seeing something I had never seen before.
The reason to stay at CC is the shore diving especially at night. All sorts of weird stuff but you have to look. Not a pretty reef like Bonaire. Lots of Boulders and sea earchins but lots of other stuff you just have to look a bit. Lot of orange ball corallamorphs(anemones?) and some really neat sea urchins. Usually a decorator crab with a couple of sizeable anemones attached to his (or her) shell. Lobsters etc.
Topside's also and adventure.
But bring your hiking boots. You have to climb some boulders if you want to get under the falls but it's worth the effort. The rain forrest is lush and almost seems primordial in places. So it only fits that the diving should have lots of strange stuff. Enough of my rambling. I really like Dominica if you can't tell. Email me if you want more info.
The only thing people complain about is the food. Starchy to say the least. Have a great trip.