Snorkeling & Wind Surfing: La Entrega Bay is the best place. Rent equipment at beach or bring your own. Take a taxi ride, get round trip price. Taxi drivers WILL come back at the proper time. Don't pay until end of round trip. (There is NOT a phone at this beach!) Also excellent snorkeling at San Agustin, the last bay. This can be included easily when you take a personal bay tour.
Diving: Huatulco offers around 13 different dive sites with its interesting coral reefs and fascinating diversity of marine life. Talk to Hector Lara Ocampo or his wife Gabi at the Leeward Dive Center 958 58 72166. Hector's email is firstname.lastname@example.org.