Sting Ray City is worth seeing, the rays are everywhere. Seeing them up close is really an experience. Unfortuntely there is no snorkling on this tour.
We found the tour to Hell/Turtle Farm to be really long and boring. I remember a group snorkling somewhere near the port, but don't know how they booked.
You may want to check how many ships will be in George Town at the same time. This port has become very popular and somewhat overwhelmed by cruise ship visits.
Grand Cayman Port Schedule
