Two really good tours you may want to look into are the Atlantis submarine tours.
One is the approx. 70 foot sub that has a good number of people- I went on this outsde of a cruise, and it was wonderful.
The other Atlantis tour is extremely expensive, fits two per ride and needs to be booked in advance: but it is the experience of a lifetime. It is the 800 foot (1000 foot available) Deep Dive in one of their two former oil exploration submarines.
We booked the tour rather late but we did get our deep dive in March, despite it being booked up (there were a few slots left and we were VIP so the excursions desk tried really hard).
There is nothing more incredible than seeing that Cayman trench, with all the fish and beautiful sea life. Watch for soda cans for size ideas: you will see cans that look far too small, because they are very very far off in the distance. We bought the video of ours, too!