IMHO - with a group of 15 that may not want to sail - for a day trip to CC with snorkeling stops and lunch if you contact one of the locals they can put together two boats, theirs or combined with a friend, and get you all there together.I would suggest contacting Alfonse or his father Pete Graniel directly. Tell them how many and where you want to go and I'm sure they will accomodate.
A trip to Turneffe may be more complicated but there isn't anything that can't "happen"! cool

Have fun!