we went on the dis excursion to trunk bay, and we enjoyed it a great deal. from what i understand, the main difference between the disney excursion and doing it yourself is time: we only got an hour and a half to snorkel and enjoy the beach on the dis trip. i havent done this, but have read other posts where people have arranged the trip themselves and stated they have like 3 hours of beach/snorkel time. it is less expensive to do it yourself, but you do have to plan it, and make sure you get back on time. i have also read that "expert" divers feel that trunk bay is too crowded and over-snorkeled, they seem to reccomend hawksnest beach and cinnamon bay. personally, i saw lots of fish at trunk bay, and was surrounded by a school of beautiful bright blue fish at one point. it was a little dissapointing to have only 1 1/2 hours, if we ever go back, we will do it ourselves so we can have more time.