Its at White Sands Resort at the north end of Tres Cocos.
This Area of the Island is called 'Buena Vista'. Here is a quote from an old San Pedro Sun Article about it.'This pristine acreage between Buena Vista and Punta Arena was designated as a conservation area in Ambergris Caye's Conceptual Master Plan of 1993. This "land" was also designated a site for protection in a Wildlife Sanctuary Expansion and Management Plan proposed by Green Reef, the local marine conservation group.'http://sanpedrosun.net/old/00-321.html
The article should have mentioned its Mangrove Swamp on the west side of the North/South road.
The cute little lagoon is however on the East side of the road.