I'd expect you would see more big stuff in T&C, especially sharks. I wasn't impressed at all with the diving around St Kitts, and Saba didn't go well for us. (They had recently dredged the harbor at Saba and much area around it was covered in silt, conditions were poor and didn't allow much diving further from the harbor, and the mooring was failing on the only really good site we did so we couldn't do it again.)
I haven't been on T&C Explorer but I believe it's a much nicer boat. Caribbean Explorer II is an older boat without as much space and I found it uncomfortable, particularly because there is no interior dining room or lounge. Part of the upper deck is used as the dining room and enclosed with plastic, supposedly has AC but they didn't seem to use it so meals were always sticky, with you stuck to the vinyl seats. Rest of the upper deck was the sundeck. So not much place to hang out besides your cabin. (Made worse on our trip because there were 4 obnoxious drunks on the boat.) Crew was great, and we didn't crash into the island when a mooring broke, so there's that.)
Presumably Saba has gotten better, and that's what attracts people to the itinerary, besides cheap. But I don't think the boat has changed, so a matter of you get what you pay for with the boat.
