The wife and I have done 12 cruises. Mostly in the Caribbean, but also the Mediterranean and one on the Yangtze River. We have cruised on Carnival, Royal Caribbean, Celebrity, Dolphin and Costa that I remember. While all of them have been really great, we keep going back to Carnival. All of the cruise lines have comparable food, you would have a hard time telling the difference between the them. Most of the ships hit the same ports and the same excursions are available. BTW, we always book our own scuba trips with independent dive shops to avoid the over crowded boats. The level of service is virtually indistinguishable between the lines. We keep going back to Carnival because we like the younger crowds. They had a reputation, some years ago, of being a "booze cruise", I haven't seen that on any of my cruises. Yes their on board activities can be somewhat juvenile at times, belly flop contest, hairy chest contest etc. You don't have to participate and there is almost always an alternative activity.
Having been on other lines and seeing the type of people that are departing on the higher end lines, we prefer the less laid back atmosphere of Carnival. It has never been what I would call rowdy, more like fun loving and adventurous. It works for us. The balcony is the way to go if you can afford it. Not only is it romantic, you can dry your gear between ports.
Please take at least your regs and mask. We also take our fins as the rental ones are usually junk. If we are diving before or after the cruise we always take our full kits. Southwest Airlines allows two bags free. Have a great time no matter who you choose and remember, you make your own fun.
RichH