You should be able to negotiate this with almost any taxi driver on the main street once you exit the ferry pier. There will be a person at the head of the taxi line who can direct you to a driver who speaks English (if you need). Tell them what you want and ask for an all inclusive price.

The other option, is to take a taxi to one of the beach clubs for your snorkeling and lunch. When finished engage another taxi from the beach club (they are always in the parking lots) to take you for a drive around the island. The snorkeling is best on the west side of the island - not on the East (or "wild" side).

Consider lunch and snorkeling on the North end of the island -- the best reefs close to shore. You could do that at Buccanos beach club or a resort such as Hotel B or Coral Princess. We love Buccanos. Its only about 5 minutes from the ferry pier.

Then ask them for a driver to take you on an island tour if you still want. Drive around to Coconuts and have a drink on the hill overlooking the beach and ocean. Really pretty. The driver will wait. You can roam the beach (but the swimming isn't great). We rarely find shells on the wild side, so its really just for the view. Some may tell you that Punta Sur (on the south end of the island) is the place for snorkeling & lunch, but I go every winter and its a long hard swim. If there is any wind, its nearly impossible to get out to the reef. You're much better off on the North.

http://www.buccanos.com/