I worked on St Maarten for 6 months and I wouldn't say it is the best diving in the world, but the Proselyte Reef is very nice and worth a couple dives. The surface conditions routinely suck... average 3-4' chop, but the reef is very vibrantly alive (or was 6 months ago when I was last there)
If you have any extra time on St Maarten... as in if it is a terminal stop for your cruise... try to get over to Saba. If you are that close to Saba, you HAVE to try!!! The diving there is seriously WORLD class, and it is only a 40 minute ferry ride away.
Either way, I recommend you speak with Vinny at
Aqua Mania - ...just add water! He is the ONLY person there who REALLY, TRULY knows everything about both Saba and St Maarten. I know ALL the operators on the Dutch side, and he is one of only two operators I would trust sending a friend or family out with. If you want, I will give you his personal email... just PM me
I CANNOT EMPHASIZE ENOUGH to make whatever efforts you can to dive on Saba. You will thank yourself. I have worked on 2 different caribbean liveaboards, and the sites on Saba were arguably the best of either itinerary. When the Viz is good to excellent, the diving there really is all that.
Please keep in mind that ALL dive operators who cater to the cruiseships tailor their trips to the clientele. This means that MOST offered on or through the cruise lines will be McDonalds style... but the one major benefit they do have is that some cruise lines may delay departure if an excursion booked through them is late. If you go on your own and for what ever reason you cannot get back to the boat on time, you are in the poop big time. I have heard of people getting left behind and having to fly to meet the boat at the next stop.
Best of luck... and enjoy the trip