Lady Sharon is based out of Cucumber Beach and indeed is servicing the Cruise Ships. We saw it six months or so ago while at Old Belize and it has taken a real beating now it's not owner operated. A bit too big for this group perhaps, even if it is an option, as it was licensed for over 100 passengers I think.
Winnie Estelle would be perfect for this trip, if not booked. I have been on a trip with similar if not slightly larger group and it was great, with tons of space, a toilet, shaded deck area if required and a nice bar area at the stern. It might be an idea to make it a bit more cost effective - not to mention a lot of fun - to take it out for the day from late morning as a snorkel trip and then return after sunset.