We just did this excursion two days ago. We also enjoyed it very much. As said earlier...the boat is fantastic, and plenty of comfortable seating. Getting on the boat was very easy to the first level, which has indoor and outdoor seating. There's a set of stairs to the second level - not particularly difficult. Entry into the water is via a few stairs that lead to a ladder into the water. OUr daughter has some mobility issues (fear of being off balance, general weakness), but had no trouble at all. There were 60 passengers, but didn't feel crowded at all.
We can't speak to the snorkeling quality, since we chose to just swim. But some on our excursion expressed disappointment by the lack of sea life. We could see that there was LOTS of coral to be seen, even without snorkel gear. And the water was fabulous. The guides provided pool noodles to float around with, life jackets if desired, and a couple of paddle boards.
We absolutely recommend this. We booked through NCL, since we didn't see any non-ship opportunities to book. Interestingly, we were mixed with a group from Celebrity, and they charged $40 more than we paid on NCL!