Travelin man is correct but be prepared to pay more than 100.us to just get there. No road to it, the only way in is by boat that cost big bucks because of high fuel cost on the island.
In reality, because of the islands typography it has no nice beaches regardless of what marketing and public relations hype would have you believe. Because of the reef being in front of the island the waves break out there and the shoreline of Ambergris Caye is the shore of an inter lagoon. Tranquility Bay is where the reef begins to meet the shore line and the inter lagoon is waning at that point. Locals joke and say 'God said you can have Reef or you can have Beaches and we said Reef'.