Hi Rick,
Just came back from a wonderful liveaboard trip with Explorer Adventures on their Explorer II boat. In one word: the crew, the food (thanks to cook Stan), the boat: perfect. I absolutely would advise you to go with Explorer Adventures, although I also have read some negative reviews on their liveaboards in TCI. Our experiences were only positive, every day.
About the diving: we went to TCI because we read in different diving magazine that the diving would be amazing. In comparison to Red Sea, Asia, Maldives, Micronesia etcetera I would say: it is good diving, but certainly not very special. As all of the divesites in the Caribean it is not so colourful as in Asia. We saw some reefsharks every dive, even one hamerhead in French Cay, but for instance I haven't seen nudibranches and bigger scorpionsfishes (only one little cute juvenile scorpionfish of 0.5 inch at a nightdive ... but you should have an eye for that).
Nevertheless we had a great time on board and if you intend going to TCI, go Explorer Adventures!
Best regards from the Netherlands,
Margriet