We returned from our trip yesterday and I thought I would report back. The guidance folks provided here was both helpful and accurate. The life vest did not detract from the experience or from the opportunity to take pictures (granted, not a big focus for me), nor, with the judicious use of rash guards and draped towels, was the lack of sunscreen any problem. From what I observed (both on our charter and on neighboring boats), the rules (and thus the sharks) were carefully respected. There were a lot of whale sharks and they seemed perfectly comfortable with the boats and swimmers, frequently approaching and swooping under and around the boats.
It was worth doing at least once; wouldn't take much to persuade me to do it again.
Thanks to all for the help!
Glad that SB answers were helpful and that you enjoyed your experience. The topic of snorkeling excursions to see anything in particular or shark feeds creates lots of SB chatter. In the end, each decides on their interest and tolerance. Shark feeds may be a bit more controversial due to the current experiences of more "active" shark encounters, but in this case, just glad the SB comments were helpful and that you enjoyed.
Rob