In GC we snorkeled with Native Way Watersports.
They pick you up at the cruise terminal. It was FABULOUS. Small groups of people and they take you to Stingray City + 2 excellent snorkeling sites. Price was on $30 per person and they accept Visa & MasterCard. Sharon is great at e-mailing back very quickly.
Check them out you won't be disappointed.
www.nativewaywatersports.com