We haven’t been on Royal Caribbean. We have been on Carnival, Norwegian Cruise Lines and Princess Cruises. We prefer Princess at this point. The dives were all boat dives (with the exception of one of the dives in St Lucia - but it was the most awesome shore dive I have been on yet). Booking on your own only saves about $20 per dive day. So, guess I am saying that it is better, if available, to dive with the cruise line.
Also, since your g/f doesn’t dive, be sure to check when you book if she is even allowed to go along on the dive boat. My wife doesn’t dive, but she was able to go along on two of the trips as a "passenger only."