I did it several years ago and really enjoyed it.
For the "snorkeler version" they have two type trips.
1.) one is Sting Ray City only (sand bottom you stand in on sandbar + stingrays)
2.) the other is Sting Ray City plus a side trip to a snorkel area with fish/reef/etc.
The trip is simple.
They pick you up in a mini-bus and take you to a dock area. They then load you on a boat, such as a 50' pontoon boat with 75 other people. You get given a snorkel mask, snorkel, and a blow up snorkel inflation vest.
You enjoy a short 10-15 minuted ride out to the sandbar. You then get off and stand i the water (waist to chest deep) and the stingrays come and run into you and then turn upwards or sideways up your sides, seeing if you have any squid in your hands. I'd say we stayed for 30-45 minutes (guess). You won't wear snorkel fins as the fins on the people will only stir up the sand and "kick" each other and the sting rays.
if you're going on a reef snorkel trip as part of the tour, you'll get snorkel fins then. Our reef was the one just north of Sting Ray City.
There's several ways to do it. One is the Sandbar
snorkel with you and 200 of your closest friends. More than that when the cruise ships are in town.
Yes it REALLY is that crowded with that many people.
Look at all the boats there that can be seen from Google Maps.
Cayman Islands - Google Maps